Make it possible to disable the fallback shaper at configure time

The OT shaper supersedes the fallback shaper in every case
and the latter become an extra weight for 99.9% of users.
diff --git a/configure.ac b/configure.ac
index 5c52c89..517b8df 100644
--- a/configure.ac
+++ b/configure.ac
@@ -127,6 +127,12 @@
 fi
 AM_CONDITIONAL(HAVE_OT, $have_ot)
 
+have_fallback=true
+if $have_fallback; then
+	AC_DEFINE(HAVE_FALLBACK, 1, [Have simple TrueType Layout backend])
+fi
+AM_CONDITIONAL(HAVE_FALLBACK, $have_fallback)
+
 dnl ===========================================================================
 
 AC_ARG_WITH(glib,