libposix4win.h

windows.h must be included after winsock2.h, ws2tcpip.h and tlhelp32.h. You also forgot to include stdint.h.

There is no check for _MSC_VER so msvclibx.h will be always be included even though it's not part of the source with ENV_MINGW.

The same for pthreads4w headers, as I tell you MSYS2 doesn't need pthreads4w: #100

There are also no check for _MSC_VER so pthreads4w headers are always included. Don't you feel it silly? Imagine you are on Cygwin where pthreads is native, you still have to use pthreads4w?

[gozfree]

msys2 env will follow linux makefile