Fix build failure when compiler.include_dirs refers to nonexistent directory

On my Windows system, Pillow fails to build on Python 2.x with the following error:
```
running build_ext
error: [Error 3] The system cannot find the path specified: 'X:\\Python27-x64\\PC/*.*'
```
The directory `'X:\Python27-x64\PC` listed in `compiler.include_dirs` is nonexistent.