build: use AX_IS_RELEASE() and AX_COMPILER_FLAGS()

The autoconf macros AX_COMPILER_FLAGS_{CFLAGS|GIR|LDFLAGS} test
for compiler and linker support of various flags, and add the flags to
the generated output.

If the command-line option '--enable-compile-warnings' is specified to
'configure', a number of additional warning options is also added to the
output. This is the default.

This update requires the presence of the GNU autoconf-archive in the
system.

plugins/Makefile.am

@@ -6,6 +6,7 @@ include $(top_srcdir)/gtester.make
 ################################################################################
 
 AM_CFLAGS = \
+	$(WARN_CFLAGS) \
 	$(MM_CFLAGS) \
 	$(CODE_COVERAGE_CFLAGS) \
 	$(GUDEV_CFLAGS)	\
@@ -22,6 +23,7 @@ AM_CFLAGS = \
 	$(NULL)
 
 AM_LDFLAGS = \
+	$(WARN_LDFLAGS) \
 	$(MM_LIBS)               \
 	$(CODE_COVERAGE_LDFLAGS) \
 	$(GUDEV_LIBS)            \