graphics/gifsicle: flavorize (x11 as default and nox11)

It turns out there are conflicting interests between desktop and
server users of this port. Flavoring the port will solve the issue by
creating packages with x11 dependency and without.

PR: 259715, 269274
Reported by: kennedy.greg@gmail.com, 0mp
Reviewed by: 0mp
MFH: 2023Q1