Unset NO_PYTHON for --with-python without arguments, and when
PYTHON_PATH is set. Do not check for PYTHON_PATH if it is set
via --with-python=PYTHON_PATH. Prefer "python" over version
specific names such as "python2.4".
Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <junkio@cox.net>