Makefile: allow PYTHON variable to override python version.

Makefile: allow PYTHON variable to override python version.

Currently, the Makefile assumes the python command that should be used
is the default python version -- the "python" executable that is found
in PATH. Compiling and testing with a different python version is not
possible without either having a system with another default version, or
by manually changing the link found in PATH.

Correct this situation by using a variable for the python command name,
that can be overridden on the command line like the following:

    make PYTHON=python2.6 test

Signed-off-by: Gabriel Filion <lelutin@gmail.net>