Removes PKG_CONFIG_LIBDIR, overrides PKG_CONFIG_PATH

As stated here => http://www.freedesktop.org/wiki/Software/pkg-config/CrossCompileProposal/

It's causing problems when using cmake pkg_check_modules and pkg-config in general as it only looks in:

```
/usr/lib/pkgconfig /usr/local/Library/ENV/pkgconfig/10.9
```