plplot forgets about install prefix while searching for self

There's a mess in cmake infrastructure if CMAKE_INSTALL_FOO_DIR is 
relative or not. Trying to fix it.

--- plplot-5.13.0/cmake/modules/instdirs.cmake.~1~	2017-08-26 05:43:25.000000000 +0000
+++ plplot-5.13.0/cmake/modules/instdirs.cmake	2018-09-07 18:45:27.925109366 +0000
@@ -34,6 +34,8 @@
 # locations below (without the CMAKE_INSTALL prefix) that are directly
 # used for installations.
 
+INCLUDE(GNUInstallDirs)
+
 set(INSTALL_LOCATION_VARIABLES_LIST)
 set(
   CMAKE_INSTALL_EXEC_PREFIX
@@ -99,56 +101,56 @@
 
 set(
   DATA_DIR
-  ${CMAKE_INSTALL_DATADIR}/${PACKAGE}${PLPLOT_VERSION}
+  ${CMAKE_INSTALL_FULL_DATADIR}/${PACKAGE}${PLPLOT_VERSION}
   CACHE PATH "PLplot install location for architecture-independent data"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST DATA_DIR)
 
 set(
   LIB_DIR
-  ${CMAKE_INSTALL_LIBDIR}
+  ${CMAKE_INSTALL_FULL_LIBDIR}
   CACHE PATH "PLplot install location for object code libraries"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST LIB_DIR)
 
 set(
   INCLUDE_DIR
-  ${CMAKE_INSTALL_INCLUDEDIR}/${PACKAGE}
+  ${CMAKE_INSTALL_FULL_INCLUDEDIR}/${PACKAGE}
   CACHE PATH "PLplot install location for C header files"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST INCLUDE_DIR)
 
 set(
   BIN_DIR
-  ${CMAKE_INSTALL_BINDIR}
+  ${CMAKE_INSTALL_FULL_BINDIR}
   CACHE PATH "PLplot install location for user executables"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST BIN_DIR)
 
 set(
   DRV_DIR
-  ${CMAKE_INSTALL_LIBDIR}/${PACKAGE}${PLPLOT_VERSION}/drivers
+  ${CMAKE_INSTALL_FULL_LIBDIR}/${PACKAGE}${PLPLOT_VERSION}/drivers
   CACHE PATH "PLplot install location for dynamically loaded devices"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST DRV_DIR)
 
 set(
   DOC_DIR
-  ${CMAKE_INSTALL_DATADIR}/doc/${PACKAGE}
+  ${CMAKE_INSTALL_FULL_DATADIR}/doc/${PACKAGE}
   CACHE PATH "PLplot install location for (architecture-independent) documentation files"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST DOC_DIR)
 
 set(
   INFO_DIR
-  ${CMAKE_INSTALL_INFODIR}
+  ${CMAKE_INSTALL_FULL_INFODIR}
   CACHE PATH "PLplot install location for (architecture-independent) info files"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST INFO_DIR)
 
 set(
   MAN_DIR
-  ${CMAKE_INSTALL_MANDIR}
+  ${CMAKE_INSTALL_FULL_MANDIR}
   CACHE PATH "PLplot install location for (architecture-independent) man files"
   )
 list(APPEND INSTALL_LOCATION_VARIABLES_LIST MAN_DIR)