plan9port: make it actually work

* Add "which" dependency, otherwise it builds with lots of
  "command not found" errors.

* -lgcc_s, otherwise many commands basically fail without pthread_cancel.

* Fix X11 dependencies, otherwise it would be plan9port with none of UI
  programs working.
Merge pull request #5245 from ftrvxmtrx/plan9port

plan9port: make it actually work