fix build for when 'python' is python3

There's also a bonus fix of a shebang that was inconsistent with the others.

It would be really nice if OS X and Debian included the python2 symlink per [PEP 394](http://www.python.org/dev/peps/pep-0394/), but sadly that's not the case. The nasty thing about this bug is that it's going to keep coming back as long as those files have a shebang that works on some systems but not others. Testing for this bug is as simple as making the `mk/*.py` files non-executable before building - is that something that could be done on the buildbot?