graphics/gmic: Restore gmic executable

... that got removed from the port after the upstream refactored
the build and switched to GNU Make. Special target 'cli' should
be added for gmic executable to be built.

Reported by: Andrea Venturoli <ml@netfence.it>

(cherry picked from commit 0cc44b495ddff468691ec05fc84e53a59c11635d)