Fix build issues in virtualenv on python3.3 and Ubuntu raring

As mentioned in #292, Pillow doesn't build on python3.3 and Ubuntu raring. libImaging fails with 
```/usr/include/python3.3m/Python.h:8:22: fatal error: pyconfig.h: No such file or directory```

In Python 3.3 platform-specific and general includes can be split in two folders. These can be determined with `syscofig` or `distutils.sysconfig`. Unfortunatelly, virtualenv has a bug, which prevents usage of the appropriate `sysconfig` methods (the returned paths are not global but virtualenv-specific). See https://github.com/pypa/virtualenv/issues/458 .

The proposed patch deals with both issues.