Add missing maths library.

diff -ru aribb24-1.0.3.orig/configure.ac aribb24-1.0.3/configure.ac
--- aribb24-1.0.3.orig/configure.ac	2016-08-06 12:04:51.260806518 +0300
+++ aribb24-1.0.3/configure.ac	2016-08-06 13:45:56.424867068 +0300
@@ -21,6 +21,12 @@
 
 AC_SUBST([PKG_REQUIRES], [$(test x$enable_shared = xno && echo ${pkg_requires})])
 
+AC_CHECK_LIB([m], [sqrt], [
+  LIBM="-lm"
+  LIBS="$LIBM $LIBS"
+])
+AC_SUBST(LIBM)
+
 AC_CHECK_FUNCS([vasprintf])
 
 AC_CONFIG_FILES([Makefile src/aribb24.pc])
diff -ru aribb24-1.0.3.orig/src/aribb24.pc.in aribb24-1.0.3/src/aribb24.pc.in
--- aribb24-1.0.3.orig/src/aribb24.pc.in	2016-08-06 12:04:51.260806518 +0300
+++ aribb24-1.0.3/src/aribb24.pc.in	2016-08-06 12:05:05.406806659 +0300
@@ -15,3 +15,4 @@
 Requires: @PKG_REQUIRES@
 Cflags: -I${includedir}
 Libs: -L${libdir} -laribb24
+Libs.private: @LIBM@