package/perl-gd: Add a new option to set the path to gdlib-config

The build of perl-gd use the gdlib-config from the host that in result
incorect features enabled, incorrect header and library paths.

Should fixes:
http://autobuild.buildroot.net/results/15c/15ce69b916167305aba2820a157c880e487ee8c7/
http://autobuild.buildroot.net/results/504/504d64d992eb504e02c127feb4734a917ad95d0a/
http://autobuild.buildroot.net/results/551/5512213c8861e51b390c1c223d53add0ec1ed758/



And many more.

Note:
In the generated Makefile, LDFLAGS still contains "-L/usr/local/lib"

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Tested-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>