This is a guide to explain various useful variables in Userland component
Makefiles.  To distinguish these from the Makefile(s) that are part of each
component distribution, the latter will be referred to as native Makefiles.

The following are the basics that just about every Makefile should have.
* COMPONENT_NAME is typically a short name (e.g., vim).
* COMPONENT_VERSION is typically numbers separated by dots (e.g. 7.3).
* COMPONENT_SRC is where the archive is extracted.  A common value for this is
  "$(COMPONENT_NAME)-$(COMPONENT_VERSION)".
* COMPONENT_PROJECT_URL is the general web site for the component.
* COMPONENT_ARCHIVE is the base name of the archive to be downloaded.  A common
  value for this is "$(COMPONENT_SRC).tar.gz".
* COMPONENT_ARCHIVE_HASH is typically "sha256:" followed by the first output
  field of `sha256sum $(COMPONENT_ARCHIVE)`.
* COMPONENT_ARCHIVE_URL is where the archive can be downloaded from.  This is
  typically constructed from $(COMPONENT_PROJECT_URL) and $(COMPONENT_ARCHIVE).
* COMPONENT_SIG_URL is the URL where the PGP signature for $(COMPONENT_ARCHIVE)
  can be found.  This can be used in addition to the hash in
  $(COMPONENT_ARCHIVE_HASH) to verify the correctness of the archive.  If
  COMPONENT_SIG_URL is present, then COMPONENT_ARCHIVE_HASH needn't be, but its
  presence is strongly encouraged to ensure that the archive contents don't
  change silently.  Note that when merging, because
  $WS/tools/.gnupg/pubring.gpg is a binary file, you will have to choose
  the correct version. To check if key is imported, run:
	gpg2 --homedir=$(git rev-parse --show-toplevel)/tools/.gnupg --list-keys 
  before you 'git commit' your merge.

These two are both initialized in make-rules/shared-macros.mk rather than any
component-level Makefile, but are frequently referenced from the latter.
* COMPONENT_DIR is the top-level directory of the given component in question.
* SOURCE_DIR is set to $(COMPONENT_DIR)/$(COMPONENT_SRC).

Additional pre/post configure, build, or install actions can be specified in
a component Makefile by setting them in one of the following macros.  None of
these have default values.  These are mostly used for miscellaneous set-up or
clean-up tweaks as their names suggest.
* COMPONENT_PRE_CONFIGURE_ACTION is used by several components to clone a
  source directory.
* COMPONENT_POST_CONFIGURE_ACTION
* COMPONENT_PRE_BUILD_ACTION
* COMPONENT_POST_BUILD_ACTION
* COMPONENT_PRE_INSTALL_ACTION
* COMPONENT_POST_INSTALL_ACTION
* COMPONENT_PRE_TEST_ACTION
* COMPONENT_POST_TEST_ACTION

If component specific make targets need to be used for build or install or
test, they can be specified via the following.
* COMPONENT_BUILD_TARGETS is not usually set because the default target of most
  open source software is the equivalent of a 'build' target.  This needs to be
  set when building the software requires a different target than the default.
  You should not override make macros here, but in COMPONENT_BUILD_ARGS.
* COMPONENT_INSTALL_TARGETS has a default value of "install".  Very few
  components need to alter this.
* COMPONENT_TEST_TARGETS has a default value of "check".  Several components
  need to set this to "test".

* COMPONENT_BUILD_ARGS is probably the mostly useful variable here for solving
  subtle build issues.  When you need to override a MACRO set in the native
  Makefile of a component, do so by adding something like:
     COMPONENT_BUILD_ARGS += MKDIR="$(MKDIR)"
  Quoting is often important because values with white-space can be split up,
  yielding the wrong results.
* COMPONENT_BUILD_ENV is for when you just need to override things in the
  calling environment, like PATH.
* COMPONENT_INSTALL_ARGS is mainly used for altering target directories.
* COMPONENT_INSTALL_ENV is mainly used for altering target directories.
* COMPONENT_TEST_ARGS is little used.
* COMPONENT_TEST_ENV is mainly used for altering PATH and friends.

If your component needs to do some kind of cleanup after a "gmake test" run,
such as kill processes after doing a "gmake test" run, then this can be done
by setting COMPONENT_TEST_CLEANUP.

If you have created master test results file(s) for your component in the
COMPONENT_TEST_RESULTS_DIR directory, then in order to successfully compare
your test results against that master results file, you might need to
normalize some of the test output lines. This is done via a set of regular
expressions that are applied to the test results. There are some global
default ones in the COMPONENT_TEST_TRANSFORMS definition in shared-macros.mk,
but your component Makefile might have to += some more for specific transforms
that need to be applied to the test output for just this component.

* COMPONENT_POST_UNPACK_ACTION is for making minor alterations to the unpacked
  source directory before any patching has taken place.  It should almost never
  be used.
* COMPONENT_PREP_ACTION is used to make alterations to the unpacked and patched
  source.  It should be used with care.

* CONFIGURE_DEFAULT_DIRS should be "yes" or "no".  A value of "yes" (the
  default) will trigger the following being passed to CONFIGURE_OPTIONS as
  parameters to corresponding options.
  * CONFIGURE_BINDIR is the value for the --bindir= option.
  * CONFIGURE_LIBDIR is the value for the --libdir= option.
  * CONFIGURE_MANDIR is the value for the --mandir= option.
  * CONFIGURE_SBINDIR is the value for the --sbindir= option.
* CONFIGURE_ENV is mainly used for passing CFLAGS and other common Makefile
  variables to configure.  When should this be used as opposed to
  CONFIGURE_OPTIONS and COMPONENT_BUILD_{ARGS,ENV}?  In general, you want
  to tell configure how to build the software using CONFIGURE_OPTIONS.  But
  sometimes you need to pass values in via the calling environment.  On rare
  occasions, you still need to do things like override MACRO settings in the
  generated Makefiles with COMPONENT_BUILD_ARGS.
* CONFIGURE_LOCALEDIR is a cousin of the other *DIR variables above, but
  rarely used and hence not triggered by CONFIGURE_DEFAULT_DIRS.
* CONFIGURE_OPTIONS is extremely useful, possibly our most used "add-on"
  variable, for passing various options to configure.  These tend to vary per
  component, but --enable-foo and --disable-foo for various values of foo are
  quite common.
* CONFIGURE_PREFIX is the prefix for the various *DIR variables above.  Its
  default is "/usr"; set it if some other value (e.g., "/usr/gnu") is needed.
* CONFIGURE_SCRIPT should be set if the default "$(SOURCE_DIR)/configure" is
  unsuitable for whatever reason.

* gcc_OPT has a default value of "-O3".  Occasional bugs in the optimizer
  have been found which have required altering this to "-O2" or lower values.
  There are also gcc_OPT.$(MACH).$(BITS) versions of this available if
  greater specificity is needed.

* Variable PYTHON3_SOABI selects between two library naming schemes of
  python3 extensions: *.cpython3Xm.so ("cpython") or *.abi3.so ("abi3").
  Currently, only a few components use ABI3 compliant extensions,
  therefore, the default value is set to "cpython".

If you frequently rebuild the same code, such as when you maintain a build server
or iterate recipes for the same component, you can benefit by caching the build
products with "ccache" - so for repeated input conditions you'd get same output
objects quickly.

Being experimental, the "ccache" integration in OI-Userland build system is
disabled by default. See below for hints how to enable and optionally configure it.

* To enable the "ccache" integration in OI-Userland build system you must
  `export ENABLE_CCACHE=true` in the calling shell, or set the make-variable
  like this: `gmake ENABLE_CCACHE=true build`
* The "ccache" program supports an inverted toggle CCACHE_DISABLE, which our
  shared-macros.mk Makefile also honours. By original project's intent, if you
  execute "ccache" then you want it to do its job. If you are debugging it or
  unsure if it causes problems, you can easily temporarily disable the "ccache"
  logic by `export CCACHE_DISABLE=true` so it quickly calls the real compiler.
  So to enable the caching feature you can also `export CCACHE_DISABLE=false`.
* In case of conflicting environtment and/or makefile variables regarding this
  feature, any explicit request to disable use of "ccache" takes precedence.
* With default settings (only `ENABLE_CCACHE=true` in place), the cache and
  non-volatile configuration will be kept in your `$HOME/.ccache` subdirectory.
* To trace progress of "ccache" with your userland component build, you can
  instrument the compilation-wrapper scripts in `tools/ccache-wrap` directory
  (then `ccache -p` can be helpful to inspect actual parsing of configuration),
  and/or use `export CCACHE_LOGFILE=/tmp/ccache.log` to trace its activities.
* You can dedicate a cache directory different from the default `$HOME/.ccache`
  for example with `export CCACHE_DIR=/tmp/ccache-dir; mkdir -p $CCACHE_DIR`.
* Note: be wary of ccache's own CCACHE_DISABLE environment variable: any
  value (empty, "false" etc.) is considered a "true" setting for ccache
  booleans (and so CCACHE_DISABLE=false still disables the program, falling
  through to real compiler). This is according to the project's documentation
  and legacy (backwards compatibility), thus not accepted by upstream as a bug.
  To negate ccache boolean environment variable settings you can use their
  CCACHE_NO* counterparts, e.g. `export CCACHE_NODISABLE=anything`.
* Troubleshooting: If no files appear in the cache, verify permissions and disk
  space. Also enable the log file and/or inspect configuration with `ccache -p`
  (see above) to see details about wrapped compilations. In particular,
  "ccache" might not actually cache the build products if the file types are
  unsupported (see the `man ccache` page for options on extending the support),
  if files are detected to have dynamic contents (e.g. `__TIME__` and similar
  macros), or when a build product is comprised of several source files (like
  running `gcc -o binprog *.c`).
* You can inspect caching statistics with `ccache -s` and wipe the cache with
  `ccache -C -z`.
* For debugging or development of the ccache component itself, you can use a
  custom build for oi-userland compilation with `export CCACHE=/path/to/ccache`
* The cache directory can contain a configuration file for "ccache" program,
  which is the recommended way to provide tweaks to your setup. While exported
  environment variables (e.g. from shell profile) may work, our Makefiles do
  not make any guarantees about passing each and every possible variable into
  sub-processes. We do make a best effort to pass the variables listed above.
* See `man ccache` for more details about the program and its configuration.