[MP] Win64: Disable OpenAL for Win64 builds.
[MP] Win64: Fix incorrect type used for Win32 API
[MP] Win64: Fixed incorrect function pointer types

Fixed/corrected the function pointer types used to cast the return of
GetProcAddress. This should all really be tidied up so that the function
pointer types are typedef'd though.
[MP] Win64: Corrected some non-cross arch code

This commit corrects some code which was 32-bit specific.
[MP] Win64: Removed zlib static lib

Zlib is now compiled from source instead of linking to a static lib. This
was the easiest way to get zlib to work with Win64, and also means you
don't get the mismatched runtime lib warnings at link time with Win32
builds when not using VS2010.