Mingw 4.0 has broken headers

[vadimcn]

The latest mingw (4.0 as of this writing) has some broken headers, which break Rust build:

• Error about FE_ALL_EXCEPT in libllvm: bits/c++config.h is missing #define _GLIBCXX_HAVE_FENV_H 1. Mingw bug: https://sourceforge.net/p/mingw/bugs/2043/
• Error about WSAPOLLFD in libuv: winsock2.h is missing a definition of WSAPOLLFD structure. https://sourceforge.net/p/mingw/bugs/2049/
• Error about FILE_FLAG_FIRST_PIPE_INSTANCE in libuv: NTDDK_VERSION is not defined in winbase.h (probably a misspelling of NTDDI_VERSION), which causes FILE_FLAG_FIRST_PIPE_INSTANCE to be missing. https://sourceforge.net/p/mingw/bugs/2050/

[klutzy]

cc #8598

[klutzy]

FILE_FLAG_FIRST_PIPE_INSTANCE is fixed on mingw 4.0.3. Other two issues are still open.

[alexcrichton]

Have these all been fixed in moving to mingw-w64?

[klutzy]

Yep! (src/etc/mingw-fix-inlcude/ is still used, but it has no effect.)

[vadimcn]

Are we ready to drop support for mingw? If so, we should get rid of
mingw-fix-include.

[alexcrichton]

These aren't really much of a maintenance burden right now, so I don't think we need to invest too much effort into removing them.

I also wouldn't classify this as a bug any longer, however. If issues crop up in the future, we can remove the extra header files at that time, but for now we can say that we build on mingw but don't officially support it. Official support will be provided for mingw-w64.