package/perl-gd: provide gd options

Now that gdlib-config is gone, provide the GD options otherwise perl-gd
will assume that everything is available:

$features = 'GD_GIF GD_GIFANIM GD_OPENPOLYGON GD_ZLIB GD_PNG GD_FREETYPE GD_FONTCONFIG GD_JPEG GD_XPM GD_TIFF GD_WEBP';

Also, while at it, also make some of the dependencies as optional as
suggested by François Perrad

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Francois Perrad <francois.perrad@gadz.org> (with
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>