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>