package/nginx: fix libgd support

Change to using gdlib-config to find libgd and its dependencies.

This fixes:
 http://autobuild.buildroot.net/results/d1806140eccb1202cb8b223289510d64fb28acb6/



Signed-off-by: Martin Bark <martin@barkynet.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>