libs: g-ir-scanner: do not hardcode libtool path

https://bugzilla.gnome.org/show_bug.cgi?id=726571
diff --git a/gst-libs/gst/gl/Makefile.am b/gst-libs/gst/gl/Makefile.am
index 2df0385..4e77e8c 100644
--- a/gst-libs/gst/gl/Makefile.am
+++ b/gst-libs/gst/gl/Makefile.am
@@ -193,7 +193,7 @@
 		--include=Gst-@GST_API_VERSION@ \
 		--include=GstBase-@GST_API_VERSION@ \
 		--include=GstVideo-@GST_API_VERSION@ \
-		--libtool="$(top_builddir)/libtool" \
+		--libtool="${LIBTOOL}" \
 		--pkg gstreamer-@GST_API_VERSION@ \
 		--pkg gstreamer-base-@GST_API_VERSION@ \
 		--pkg gstreamer-video-@GST_API_VERSION@ \