fixes bad path resolution in Python setup.py (zlib.h is not located into /usr/include anymore.)
Fixes for changes introduced by installing new XCode5 CommandLineTools, which put system headers in /usr/local.  This change makes sure that the hostpython is using the system sdk, and PIL does not explicitly include /usr/include etc. as the highest priority path to look for headers in...that wont work if we are cross compiling.
ensure byte-compiling py->pyo will work. closes #71