New --disable-glib configure option

This option uses the fake GLib shim even when GLib is present on the
system.

src/libmdb/Makefile.am

@@ -1,6 +1,6 @@
 lib_LTLIBRARIES	=	libmdb.la
 libmdb_la_SOURCES=	catalog.c mem.c file.c table.c data.c dump.c backend.c money.c sargs.c index.c like.c write.c stats.c map.c props.c worktable.c options.c iconv.c
-if !HAVE_GLIB
+if FAKE_GLIB
 libmdb_la_SOURCES += fakeglib.c
 endif
 libmdb_la_LDFLAGS = -version-info 2:1:0 -export-symbols-regex '^(mdb_|_mdb_put_int16$$|_mdb_put_int32$$)'