Replace -I with $(glib_INCLUDES) and friends

Stop using ad hoc -I in all of our Makefile.am.  Use the new variables
instead.

docs/reference/gio/Makefile.am

@@ -116,15 +116,7 @@ IGNORE_HFILES=				\
 # CFLAGS and LDFLAGS for compiling scan program. Only needed
 # if $(DOC_MODULE).types is non-empty.
 INCLUDES = \
-	-I$(srcdir)			\
-	-I$(top_srcdir)			\
-	-I$(top_srcdir)/glib		\
-	-I$(top_srcdir)/gobject		\
-	-I$(top_srcdir)/gio		\
-	-I$(top_builddir)		\
-	-I$(top_builddir)/glib		\
-	-I$(top_builddir)/gobject	\
-	-I$(top_builddir)/gio		\
+	$(gio_INCLUDES)			\
 	$(GLIB_DEBUG_FLAGS)
 
 GTKDOC_LIBS = \

docs/reference/gobject/Makefile.am

@@ -32,10 +32,7 @@ IGNORE_HFILES = \
 # if $(DOC_MODULE).types is non-empty.
 INCLUDES = \
 	-I$(srcdir) 			\
-	-I$(top_srcdir) 		\
-	-I$(top_srcdir)/glib 		\
-	-I$(top_builddir) 		\
-	-I$(top_builddir)/glib 		\
+	$(gobject_INCLUDES) 		\
 	$(GLIB_DEBUG_FLAGS)
 GTKDOC_LIBS = \
 	$(top_builddir)/glib/libglib-2.0.la	\