• Add missing python dependency
• Provide PYTHON_CMD to configure explicitly

Without it, configure tries to find python itself (trying python, python2,
python3, python2.0, .., python3.5) which is a) prone to finding incorrect
python executable and creating a hidden dependency and b) fails to detect
python3.6. So this change also fixes build with python3.6 as default.

Approved by: portmgr blanket