Split maintainer-flags from the compiler flags

The maintainer compiler flags we use trigger warnings and errors
in the autogenerated code that gtk-doc creates to scan the header
and source files. Since we cannot control that, and we must run
a distcheck with both --enable-gtk-doc and --enable-maintainer-flags
turned on, we need to use less-strict compiler flags when inside
the doc/reference subdirectories.

The way to do this is to split the maintainer compiler flags into
their own Makefile variable, called MAINTAINER_CFLAGS. The we
can use $(MAINTAINER_CFLAGS) in the INCLUDES or _CFLAGS sections
of each part of the source directories we wish to check with the
anal retentiveness suited for maintainers.

clutter/pango/Makefile.am

@@ -19,6 +19,7 @@ libcoglpango_la_SOURCES = \
 INCLUDES = \
 	@GCC_FLAGS@ @CLUTTER_CFLAGS@ 			\
 	$(CLUTTER_DEBUG_CFLAGS)				\
+	$(MAINTAINER_CFLAGS)				\
 	-DCLUTTER_COMPILATION                           \
 	-I$(top_srcdir)	                 		\
 	-I$(top_srcdir)/clutter				\