libgtk3: ensure cups-config can be found

Fixes:

http://autobuild.buildroot.net/results/db6/db6cd8922444490056bf5157fc0c9349e9bf383b/
http://autobuild.buildroot.net/results/4bd/4bdcd642eaa880dbbfbed8f4ed5ffac0c52edaa8/
http://autobuild.buildroot.net/results/8d3/8d30687f92178de15d7697a7b8a1b7e2d6e93deb/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libgtk3/libgtk3.mk

@@ -96,6 +96,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_CUPS),y)
 LIBGTK3_CONF_OPTS += --enable-cups
+LIBGTK3_CONF_ENV += ac_cv_path_CUPS_CONFIG=$(STAGING_DIR)/usr/bin/cups-config
 LIBGTK3_DEPENDENCIES += cups
 else
 LIBGTK3_CONF_OPTS += --disable-cups