Build environment for official win32 builds

[Tor Lillqvist]

>> Is there some specific reason for using MSVC instead of gcc ?
>
>  Currently ABI backwards-compatibility for compiled extensions.

There are also some features in the code that don't compile with
MinGW. They use API that MinGW does not provide headers for etc.

Let me also point out that we definitely do not want to build
*locally* on Windows with GCC (MinGW). MinGW makes sense to us only
for cross-compilation. If one wants to build locally on a Windows
machine, use MSVC.

Note that when we say MinGW, we actually mean mingw-w64, a fork of
MinGW that provides a more complete set of headers and import
libraries for various Windows APIs. (Despite the "w64" in the name,
they do provide a tool-chain producing 32-bit code, too.)

--tml