Fix compilation error when using MSYS2 environment.
Credits to MSYS2 Ruby package using this patch.

https://developercommunity.visualstudio.com/t/warning-C5287:-operands-are-different-e/10877942?

It is not able to silence "operands are different enum types"
warnings, even using an explicit cast, as the message says.

```
x64-vcruntime140-ruby350.def : error LNK2001: unresolved external symbol Avx2WmemEnabledWeakValue
```

winpthreads-git 12.0.0.r720 provides `clock_gettime` and
`clock_getres` as inline functions.

Windows 11 SDK Version 10.0.26100.0 introduced a new internal inline
function in ucrt/corecrt_math.h.  Even it appears in object files and
will be included in the DEF file, it will be removed from the DLL and
result in a linker error.

Warned if both of `main` and `wmain` are exposed:
```
LINK : warning LNK4067: ambiguous entry point; selected 'mainCRTStartup'
```

	Define `incflags` also on mswin

HOMEPATH is set to "\WINDOWS\system32" when running per "runas" session.
This directory is not writable by ordinary users, leading to errors with many ruby tools.
Also config files in the home directory are not recognized.

Still keeping HOME at first which is not used by native Windows,
but by ruby specs and by MSYS2 environment.