configure: Allow --without-python

This patch allows someone to use configure to build git while at the
same time disabling the python remote helper code.  It leverages the
ability of GIT_ARG_SET_PATH to accept an optional second argument
indicating that --without-$PROGRAM is acceptable.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

configure.ac

@@ -288,7 +288,7 @@ GIT_ARG_SET_PATH(shell)
 GIT_ARG_SET_PATH(perl)
 #
 # Define PYTHON_PATH to provide path to Python.
-GIT_ARG_SET_PATH(python)
+GIT_ARG_SET_PATH(python, allow-without)
 #
 # Define ZLIB_PATH to provide path to zlib.
 GIT_ARG_SET_PATH(zlib)