Unset PYTHONPATH in the makefile
Unset PYTHONPATH in the makefile

PYTHONPATH could interfere with the build system and cause unclear
error messages like the one below:

ImportError: This package should not be accessible on Python 3. Either
you are trying to run from the python-future src folder or your
installation of python-future is corrupted.

Change-Id: Ic0a491ad70041bb8e130cbc402dc35fed1d1a095
Reviewed-on: https://gerrit.franz.com:9080/9810
Reviewed-by: Tadeusz Sznuk <4402abb98f9559cbfb6d73029f928227b498069b@franz.com>
Tested-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@franz.com>
Reviewed-by: Hudson <dfb64870173313a9a7b56f814c8e3b33e268497a@franz.com>
Reviewed-by: Ahmon Dancy <8f7d8ce2c6797410ae95fecd4c30801ee9f760ac@franz.com>

franzinc/agraph-python,franzinc/agraph-python,franzinc/agraph-python,franzinc/agraph-python