grc: added logic to discover the path to the python interpreter
Rather than simply exec-ing the application w/ "python",
use the same interpreter that executed grc w/ full path.
Added code to handle the following exceptions:
- for a wx app on mac osx, use the pythonw interpreter
(this was in the m4 file, but its easier as a runtime check)
- for a no gui app on linux, prepend xterm cuz its nice
(we were already doing that but its now restricted to linux)