gd: use pkg-config to figure out png linker flags

So gdlib-config --libs returns the full dependency chain (-lpng16 -lz -m)
when linking statically.

Fixes http://autobuild.buildroot.net/results/dac/dac3eb950c7c27b2f09f001f9db9936f897721f9/

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

package/gd/gd.mk

@@ -40,7 +40,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_LIBPNG),y)
 GD_DEPENDENCIES += libpng
-GD_CONF_OPTS += --with-png=$(STAGING_DIR)/usr
+GD_CONF_OPTS += --with-png
 else
 GD_CONF_OPTS += --without-png
 endif