[PATCH] Support python in other than /usr/bin/python

[Ed Maste]

That is to say, it seemed like this intentionally used `/usr/bin/python` if it exists, rather than relying on python in the user's path, and I tried to keep that behaviour. I'm happy with either of

- `#!/usr/bin/env python`
- A `find_python` shell function that sets PYTHON_EXECUTABLE to the first match from `/usr/bin/python`, `/usr/local/bin/python`, `$(which python)`


http://reviews.llvm.org/D9914

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