core: use GLIB_VERSION_MAX_ALLOWED=2.28

We want to build/run even on relatively old OSes, so that we can
be used to bootstrap and maintain newer builds.

Makefile.am

@@ -16,7 +16,8 @@
 # Boston, MA 02111-1307, USA.
 
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
-AM_CPPFLAGS = -DDATADIR='"$(datadir)"' -DLIBEXECDIR='"$(libexecdir)"'
+AM_CPPFLAGS = -DDATADIR='"$(datadir)"' -DLIBEXECDIR='"$(libexecdir)"' \
+	-DGLIB_VERSION_MIN_REQUIRED=GLIB_VERSION_2_28 -DGLIB_VERSION_MAX_ALLOWED=GLIB_VERSION_2_28
 AM_CFLAGS = $(WARN_CFLAGS)
 
 NULL =