The installation needs to use python2, but it uses python in the
Makefile which fails later with syntax error
(since it is written for python2).

As per PEP-394 - https://www.python.org/dev/peps/pep-0394/ -,
'all distributions should ensure that python refers to the same target
as python2 .', hence this change.

Therefore, python2 is used by default in lib/Makefile, though
this can be overriden with PYTHON_BIN.

Tested with:

a) PYTHON_BIN=python2 make install
b) make install

c) export PYTHON_BIN=python2
   make test

Signed-off-by: Raghavendra Prabhu <me at rdprabhu dot com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>