Using find_package() for Python rather than find_program
Using find_package() for Python rather than find_program

This allows the build to work properly for OSes with nonstandard names for,
e.g., the Python 2.7 executable, such as Arch Linux (python2 instead of python).

girder/girder,adsorensen/girder,jcfr/girder,sutartmelson/girder,opadron/girder,salamb/girder,jbeezley/girder,sutartmelson/girder,RafaelPalomar/girder,girder/girder,jbeezley/girder,salamb/girder,essamjoubori/girder,chrismattmann/girder,data-exp-lab/girder,opadron/girder,salamb/girder,Kitware/girder,Kitware/girder,jcfr/girder,kotfic/girder,kotfic/girder,RafaelPalomar/girder,essamjoubori/girder,msmolens/girder,kotfic/girder,jcfr/girder,opadron/girder,manthey/girder,salamb/girder,chrismattmann/girder,data-exp-lab/girder,RafaelPalomar/girder,jcfr/girder,adsorensen/girder,Kitware/girder,manthey/girder,data-exp-lab/girder,Xarthisius/girder,girder/girder,jcfr/girder,RafaelPalomar/girder,chrismattmann/girder,msmolens/girder,sutartmelson/girder,data-exp-lab/girder,manthey/girder,chrismattmann/girder,Xarthisius/girder,adsorensen/girder,manthey/girder,sutartmelson/girder,essamjoubori/girder,chrismattmann/girder,essamjoubori/girder,msmolens/girder,Kitware/girder,sutartmelson/girder,kotfic/girder,msmolens/girder,essamjoubori/girder,jbeezley/girder,salamb/girder,Xarthisius/girder,jbeezley/girder,kotfic/girder,Xarthisius/girder,girder/girder,opadron/girder,opadron/girder,data-exp-lab/girder,adsorensen/girder,Xarthisius/girder,RafaelPalomar/girder,msmolens/girder,adsorensen/girder