Windows pkgconfig fix

This corrects the zlib.pc file for the MinGW build so that it correctly references the library and include folders.

Before this fix `zlib.pc`:

```
prefix=/the/installation/path
exec_prefix=/the/installation/path
libdir=/path/specified/as/LIBRARY_PATH
sharedlibdir=/path/specified/as/LIBRARY_PATH
includedir=/path/specified/as/INCLUDE_PATH
```

After the fix `zlib.pc`:

```
prefix=/the/installation/path
exec_prefix=${prefix}
libdir=${prefix}/path/specified/as/LIBRARY_PATH
sharedlibdir=${prefix}/path/specified/as/LIBRARY_PATH
includedir=${prefix}/path/specified/as/INCLUDE_PATH
```

This allows `pkgconfig` to correctly find the files.