autoconf: provide a harfbuzz-config for use from CMake (#822)

diff --git a/configure.ac b/configure.ac
index 8c3ab20..eee2568 100644
--- a/configure.ac
+++ b/configure.ac
@@ -184,6 +184,7 @@
 	AC_SUBST(GLIB_MKENUMS)
 fi
 AM_CONDITIONAL(HAVE_GOBJECT, $have_gobject)
+AC_SUBST(have_gobject)
 
 dnl ===========================================================================
 
@@ -491,6 +492,7 @@
 Makefile
 src/Makefile
 src/hb-version.h
+src/harfbuzz-config.cmake
 src/hb-ucdn/Makefile
 util/Makefile
 test/Makefile