Med: Use generic USE_GHASH_COMPAT macro instead of autoconf's HAVE_LIBGLIB_2_0

cs:e75a2b5910f9 added g_hash_* functions for compatibility with
glib < 2.14, but the HAVE_LIBGLIB_2_0 macro used to disable these
in include/crm/common/util.h is only set internally in Pacemaker's
autoconf/configure. This is (obviously) fine for Pacemaker, but
means that other packages building against Pacemaker's headers
will always have the compatibility functions included, which will
break those builds on systems with glib >= 2.14. This change
abstracts that macro up a layer, setting USE_GHASH_COMPAT in
crm_config.h, instead of directly using the autoconf macro in
a public header.

Mercurial revision: 76e079ae96f43b67a46d920da81975ff56fb72eb