Add configure test for garbage collector friendliness for GLib. If

Add configure test for garbage collector friendliness for GLib. If

2000-04-17  Sebastian Wilhelmi  <wilhelmi@ira.uka.de>

	* configure.in, acconfig.h: Add configure test for garbage
	collector friendliness for GLib. If enabled, ENABLE_GC_FRIENDLY
	will be defined.

	* garray.c, ghash.c, glist.c, gmain.c, gmem.c, gnode.c, gqueue.c,
	gslist.c, gtree.c: If ENABLE_GC_FRIENDLY is defined, NULLify all
	memory released by the user, but cached by GLib. This lets a
	garbage collector have a more correct view of the actually used
	memory.