[configure] Check that mutter was compiled with GTK+-3.0

If mutter was compiled with GTK+-2.0, the build will fail with a
rather cryptic message - better fail early during configure with
a proper error message.

https://bugzilla.gnome.org/show_bug.cgi?id=623889

configure.ac

@@ -154,6 +154,14 @@ changequote([,])dnl
 AC_PATH_PROG(mutter, [mutter])
 AC_SUBST(mutter)
 
+AC_MSG_CHECKING([if mutter was compiled with GTK+-3.0])
+if $PKG_CONFIG --print-requires libmutter-private | grep -q gtk+-3; then
+  AC_MSG_RESULT(yes)
+else
+  AC_MSG_RESULT(no)
+  AC_MSG_ERROR([GNOME Shell requires Mutter to be compiled against GTK+-3.0])
+fi
+
 AC_CONFIG_FILES([
   Makefile
   data/Makefile