Windows build should prefer native threads over winpthreads, or provide an option to allow it

Currently, if the MinGW toolchain contains a copy of winpthreads, configure will detect it, and use it, instead of using native win32 threads (from src/win32).

There is no way to tell configure to not do this, and there should be, or configure sould prefer this over winpthreads.

Current workaround is to remove winpthreads from the MinGW sysroot, but this is undesirable, when, for example, MinGW is a distro-provided package.