configure.ac: add support for a specified python
configure.ac: add support for a specified python

We're going to use this to allow building against custom Python versions.

facebook/watchman,wez/watchman,facebook/watchman,facebook/watchman,dhruvsinghal/watchman,wez/watchman,dhruvsinghal/watchman,dhruvsinghal/watchman,facebook/watchman,facebook/watchman,dhruvsinghal/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,kwlzn/watchman,dhruvsinghal/watchman,kwlzn/watchman,nodakai/watchman,wez/watchman,facebook/watchman,nodakai/watchman,kwlzn/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,dhruvsinghal/watchman,kwlzn/watchman,facebook/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,dhruvsinghal/watchman,wez/watchman,facebook/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,dhruvsinghal/watchman