Summary:
Add some CMake functions for building standalone executables from Python
source files. This generates executables similar to PEX
(https://github.com/pantsbuild/pex).
In the future this could potentially be leveraged to directly build XAR files
(https://github.com/facebookincubator/xar).
The main advantages of these functions is that they allow easily defining
"libraries" of python files, and their dependencies, which can then be used
and packaged as part of multiple different standalone executables.
Reviewed By: wez
Differential Revision: D16722499
fbshipit-source-id: e1d829b911dc428e5438b5cf9cebf99b3fb6ce24
