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.

common/Makefile.am

@@ -9,6 +9,7 @@ INCLUDES = \
         -DCLUTTER_COMPILATION \
 	$(CLUTTER_CFLAGS) \
 	$(CLUTTER_DEBUG_CFLAGS) \
+	$(MAINTAINER_CFLAGS) \
 	$(GCC_FLAGS)
 
 LDADD = $(CLUTTER_LIBS)

gl/Makefile.am

@@ -24,6 +24,7 @@ INCLUDES = \
         -DCLUTTER_COMPILATION \
 	$(CLUTTER_CFLAGS) \
 	$(CLUTTER_DEBUG_CFLAGS) \
+	$(MAINTAINER_CFLAGS) \
 	$(GCC_FLAGS)
 
 LDADD = $(CLUTTER_LIBS)

gles/Makefile.am

@@ -24,6 +24,7 @@ INCLUDES = \
         -DCLUTTER_COMPILATION \
 	$(CLUTTER_CFLAGS) \
 	$(CLUTTER_DEBUG_CFLAGS) \
+	$(MAINTAINER_CFLAGS) \
 	$(GCC_FLAGS)
 
 LDADD = $(CLUTTER_LIBS)