Currently we use AC_CHECK_PROGS looking for python2.7, python2 and
finally python. That is due to the varying names used across the
different OS.

Use the handy AM_PATH_PYTHON which finds the correct name and checks for
the version.

Note: python2.7 has been an unofficial requirement for quite some time.
Update the docs to reflect that.

Cc: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>