Building IBPP on MinGW-w64

When building IBPP on MinGW-w64 MinGW complains that "Only Win32 is supported." Hey, I'm on Windows and we're building a Win32 executable!

But well, don't forget we're using an open source project and we're free to review the source code. Let's see _ibpp.h to find out why:

#if (defined(__GNUC__) && defined(IBPP_WINDOWS))
// UNSETTING flags used above for ibase.h -- Huge conflicts with libstdc++ !
#undef _MSC_VER
#undef _WIN32
#endif

As you see, if the compiler is found to be MinGW it #undefs _WIN32. It seems to be because of the compatibility against MinGW 3.0, yet in MinGW-w64 this is the very reason to block the "normal" build.

Hope this would help any Firebird users, who want to use it with IBPP under MinGW-w64.

P.S:
For relations and differences between MinGW and MinGW-w64, please refer to the URLs below:
https://sourceforge.net/p/mingw-w64/wiki2/History/
https://sourceforge.net/p/mingw-w64/wiki2/Feature%20list/