Use newer glib-genmarshal to simplify build rules

The glib-genmarshal tool from GLib 2.54 added various command line
arguments that allow us to remove a bunch of as hoc manipulations of
the generated marshaller source files. The marshal generator tool can
now include an header in the source, and undef the G_ENABLE_DEBUG
pre-processor symbol for us. It can also generate the prototypes of the
marshallers in the C source, and avoid a 'missing-prototypes' compiler
warning.

gdk/Makefile.am

@@ -426,6 +426,7 @@ gdkmarshalers.c: gdkmarshalers.list
 	$(AM_V_GEN) $(GLIB_GENMARSHAL) \
 		--prefix=_gdk_marshal \
 		--body \
+		--prototypes \
 		$(srcdir)/gdkmarshalers.list > gdkmarshalers-c.tmp	\
 	&& mv gdkmarshalers-c.tmp gdkmarshalers.c \
 	|| ( rm -f gdkmarshalers-c.tmp && exit 1 )

gtk/Makefile.am

@@ -1196,19 +1196,21 @@ gtkmarshalers.h: stamp-gtkmarshalers.h
 	@true
 stamp-gtkmarshalers.h: gtkmarshalers.list
 	$(AM_V_GEN) $(GLIB_GENMARSHAL) \
-		--prefix=_gtk_marshal \
 		--header \
+		--prefix=_gtk_marshal \
 		--valist-marshallers \
-		$(srcdir)/gtkmarshalers.list >> xgen-gmlh \
+		$(srcdir)/gtkmarshalers.list > xgen-gmlh \
 	&& (cmp -s xgen-gmlh gtkmarshalers.h || cp xgen-gmlh gtkmarshalers.h) \
 	&& rm -f xgen-gmlh \
 	&& echo timestamp > $(@F)
 gtkmarshalers.c: gtkmarshalers.list
-	$(AM_V_GEN) (echo "#undef G_ENABLE_DEBUG"; \
-	 $(GLIB_GENMARSHAL) \
-	 	--prefix=_gtk_marshal \
+	$(AM_V_GEN) $(GLIB_GENMARSHAL) \
 		--body \
-		--valist-marshallers $(srcdir)/gtkmarshalers.list) >> xgen-gmlc \
+		--prefix=_gtk_marshal \
+		--valist-marshallers \
+		--include-header="gtkmarshalers.h" \
+		-U G_ENABLE_DEBUG \
+		$(srcdir)/gtkmarshalers.list > xgen-gmlc \
 	&& cp xgen-gmlc gtkmarshalers.c \
 	&& rm -f xgen-gmlc