manage the ports options in make.conf

instead of keeping verbatim copies of each options file.

this allows us to only keep and set those options that are non-default. in this commit we're still touching more than are needed but it's a first step.
don't deactivate gtk3 for libreoffice

apparently we need to select at least one of the graphical environments for the build to succeed. going with gt3 as a random attempt to get it to work. no other reason for chosing it over the others.