invoke empy with specific python interpreter

[dirk-thomas]

@tfoote @wjwwood Please review.

[wjwwood]

+1 looks reasonable to me, though I haven't tested it.

[wjwwood]

On my machine it is finding the .pyc file for em:

-- Found PythonInterp: /usr/bin/python (found version "2.7.5")
-- Using PYTHON_EXECUTABLE: /usr/bin/python
-- Using default Python package layout
-- Found PY_em: /Library/Python/2.7/site-packages/em.pyc
-- Using empy: /Library/Python/2.7/site-packages/em.pyc

Everything seems to build fine though.

[tfoote]

+1