Fix HAVE_INTROSPECTION does not appear in AM_CONDITIONAL

During autoreconf GOBJECT_INTROSPECTION_CHECK could not be resolved because we
don't have introspection which provides its custom m4 macro. Reconfigure fails
with:

gdk/Makefile.am:196: error: HAVE_INTROSPECTION does not appear in AM_CONDITIONAL
gtk/Makefile.am:1347: error: HAVE_INTROSPECTION does not appear in AM_CONDITIONAL

We avoid to add a copy of introspection.m4 in the m4 directory of libgtk3 by
adding a check, as performed in Systemd.

Signed-off-by: Hadrien Boutteville <hadrien.boutteville@gmail.com>

--- a/configure.ac
+++ b/configure.ac
@@ -1621,7 +1621,10 @@
 # GObject introspection
 ##################################################

-GOBJECT_INTROSPECTION_CHECK(introspection_required_version)
+m4_ifdef([GOBJECT_INTROSPECTION_CHECK],
+         [GOBJECT_INTROSPECTION_CHECK(introspection_required_version)],
+         [AM_CONDITIONAL([HAVE_INTROSPECTION], [false])
+          enable_introspection=no])

 ##################################################
 # colord module