Added a Windows-friendly build option ZLIB_SOURCE.

Being inspired by Boost's ```-sZLIB_SOURCE``` option, I added a Windows-friendly build option ```ZLIB_SOURCE``` which compile and link zlib with TagLib easily.
If you have zlib's source code. You can use the option like this:
```
cmake -G "Visual Studio 12" -DZLIB_SOURCE="Z:/zlib-1.2.8" ..
```