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: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>