Makefile: detect when PYTHON_PATH changes

Makefile: detect when PYTHON_PATH changes

When make is run, the python scripts are created from *.py files that
are changed to use the python given by PYTHON_PATH. And PYTHON_PATH
is set by default to /usr/bin/python on Linux.

However, next time make is run with a different value in PYTHON_PATH,
we failed to regenerate these scripts.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>