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