Fixes Python patch step failing on OSX

Path to cmake command might include spaces.
Patches Python to remove global OSX packages

On Mac OS X only, if Python is built as a framework, site.py loads
packages from /Library/... into sys.path.

This disables it by patching site.py before installation.
Error checking for Python patch step
Merge pull request #831 from remram44/fixes-osx-global-pkgs

Patches Python to remove global OSX packages
Ok Sam confirmed it works for him.