package/nginx: drop gdlib-config

Drop gdlib-config and use pkg-config to find gd.
Indeed, gdlib.pc is available since version 2.1.0 and
https://github.com/libgd/libgd/commit/071a2a94ee9f9e5ce00ff689dfe67a0fcb885ad6


and gdlib-config has been dropped from version 2.3.0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>