python_CMD in shebangfix.mk

[Michael Gmelin]

This has been discussed before on freebsd-python:

http://lists.freebsd.org/pipermail/freebsd-python/2014-May/006964.html

I just found myself writing 

python_OLD_CMD= /usr/bin/env python
python_CMD=     ${PYTHON_CMD}

as python_CMD is /usr/local/bin/python by default, which seems to be
against

https://wiki.freebsd.org/Python/PortsPolicy

which says

"Executable scripts being installed MUST use ${PYTHON_CMD as shebang
line. See: Mk/Uses/shebangfix.mk"

So

python_CMD?= ${PYTHON_CMD} would seem a reasonable default.

Also, adding "/usr/bin/env <language>" as an alternative
<language>_OLD_CMD would be useful.

- Michael

-- 
Michael Gmelin