package/gnuplot: 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

gnuplot supports pkg-config to find gd since version 5.4.1 and
https://sourceforge.net/p/gnuplot/gnuplot-main/ci/4a64124cf5e7bf7d6fd7be1f1a7b853c022678fd



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>