Add the ability to specified the PYTHON_EXE in Makefile #54

Signed-off-by: Thomas Druez <tdruez@nexb.com>

Makefile

@@ -20,7 +20,8 @@
 # ScanCode.io is a free software code scanning tool from nexB Inc. and others.
 # Visit https://github.com/nexB/scancode.io for support and download.
 
-PYTHON_EXE=python3
+# Python version can be specified with `$ PYTHON_EXE=python3.x make conf`
+PYTHON_EXE?=python3
 MANAGE=bin/python manage.py
 ACTIVATE=. bin/activate;
 ANSIBLE_PLAYBOOK=cd etc/ansible/ && ansible-playbook --inventory-file=hosts --verbose --ask-become-pass --user=${USER}