`conflicting types` build failure on mingw-w64 32bit

[klutzy]

/path/to/rust/src/etc/mingw-fix-include/winsock2.h:10:3: error: conflicting types for 'WSAPOLLFD'

Latest mingw has broken headers (#9246) therefore #9254 added missing definitions locally. This causes error on mingw-w64 since its header is not broken. :(

cc @vadimcn

[klutzy]

Quick fix is adding #if defined(__MINGW_MAJOR _VERSION) && __MINGW_MAJOR _VERSION == 4 at winsock2.h. mingw-w64 doesn't define them, defines __MINGW64_VERSION_{MAJOR, MINOR} instead.

I think we should prepare a plan when mingw also fixes it. (They already fixed third one of #9246 on w32api-4.0.3.)
However, seems like mingw doesn't increment __MINGW_PATCHLEVEL: both 4.0.0-1 and 4.0.3 have value 0.

[vadimcn]

@klutzy: mingw-w64 gets its' own section in platform.mk, doesn't it? So adding mingw-fix-include to include path in rt.mk can be conditioned on some macro defined there.

As for mingw fixing their headers, I think we can just remove fixed headers from mingw-fix-include when that happens.

[klutzy]

@vadimcn I'm building rust on mingw-w64-32bit but rust thinks it is on mingw. I'm not sure this is "legal", but I don't want to cross-build from mingw to mingw-w64-32bit since their difference is small.

[klutzy]

platform.mk has three mingw entries: i686-pc-mingw32 (normal mingw), i586-mingw32msvc (anyone who know this?), and x86_64-w64-mingw32 (mingw-w64 64bit).
We don't have i686-w64-mingw32 (mingw-w64 32-bit). I don't know if it is good idea of adding it, so quick fix seems sufficient for now.