Makefiles: auto-detect python version - prefer python2

This prepares CRIT for python2/python3 compatibility by auto-detecting
the installed python version. python2 is detected first and then the
variable PYTHON is set.

By setting the variable PYTHON to python2/python3 the user can override
the auto-detection.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>