[PATCH] Fix paths on FreeBSD by processing gitk like other scripts

The paths for python and tk are not /usr/bin for FreeBSD, so I moved
gitk to gitk.tk and added a rule to sed in the proper path to "wish" in
making gitk, and also added the appropriate default path for python.

-- 
Eric Anholt                     anholt@FreeBSD.org
eric@anholt.net                 eric.anholt@intel.com