gtags startup issue with pygments and python3 executable

[Ingmar Sittl]

Hi,

some GNU/Linux distros have moved away from providing a plain "python" link 
since moving to python3. The configure script correctly finds that "python3" is 
the working python executable, but at least pygments_parser.py doesn't take that 
into account, which leads to an error when starting a pygments-enabled gtags.

Using @PYTHON@ instead of plain "python" in the shebang seems to fix this, at 
least gtags runs without producing further errors.

Regards,
Ingmar Sittl