graphics/darktable: resolve G'Mic inconsistency

The build will automatically pick up G'Mic if available, leading
to inconsistent builds between systems. Make it an explicit OPTION
which defaults to off, since only the lut3d module uses it and only
to read .gmz G'Mic-compressed LUT files.

Approved by:	Jean-Sébastien Pédron (dumbbell@, maintainer)