Clean up mingw-w64 project to make it clean

The pthread situation is a bit unfortunate. It doesn't seem that Rust depends on winpthread: ​https://github.com/rust-lang/rust/issues/13501 Have you changed --enable-threads=posix to --enable-threads=win32? Also you can remove old --with-gnu-ld --with-gnu-as.

LDFLAGS_FOR_TARGET does not work for some reason. Thus, we take

CFLAGS_FOR_TARGET.

It didn't work, because linker didn't want to eat -specs. Try now. Also where are --no-seh, --large-address-aware for x86 and --high-entropy-va, --image-base for x64?

--enable-sdk=all --enable-secure-api They are no longer needed (set by default).