First attempt at Windows port

CMakeLists.txt:

* Added a conditional to link WinSock on Windows - Mongoose doesn't
compile otherwise

mkdata.c:

* Changed error handling to use standard C routines - no error() on
Windows
* Changed file mode to "rb" - avoids early EOFs on null bytes on Windows

What doesn't work:

* DB browsing
* Search?

What works:

* Everything else

All tested with Fedora 20, both native compilation and MinGW(-w64), MinGW cross-compiled binaries of mpd, libmpdclient and ympd also tested on Windows 8.1 64-bit. Any ideas as to what breaks search/DB? I don't really have any established debugging workflow on Windows, so it'll take me some time to figure out how to figure it out.