science/massxpert: Update 3.4.0 -> 3.6.1

This software seems to be abandoned by its upstream, and rescued by the Debian team.
I've set its source to Debian's GitLab server, where they source it from now.

Port changes:

• 3.6.1 is Qt5-based: remove the DEPRECATED lines
• USE_GCC seems to be needed due to clang error messages
• MAKE_JOBS_UNSAFE isn't needed