Disable UCDN if glib is available

We only want UCDN if we don't have any other Unicode provider.
I'm going to remove ICU from the list of default Unicode providers
as part of moving hb-icu into its own library.  As such, the only
providers will be UCDN and glib.

configure.ac

diff --git a/configure.ac b/configure.ac
index 9ac0af5..2371537 100644
--- a/configure.ac
+++ b/configure.ac
@@ -145,6 +145,9 @@
 dnl ===========================================================================
 
 have_ucdn=true
+if $have_glib; then
+	have_ucdn=false
+fi
 if $have_ucdn; then
 	AC_DEFINE(HAVE_UCDN, 1, [Have UCDN Unicode functions])
 fi