[libcxx] Don't autodetect pthreads on MinGW

e9ee5179 added support for using
winpthreads on Windows, enabled if `__WINPTHREADS_VERSION` was
defined (i.e. if winpthreads headers have been included before
including libcxx `__config`). This was fragile (libcxx changed
behaviour depending on what headers had been included externally
before), and was changed in a1bc823a
to use pthreads on Windows whenever the pthread.h header was
available.

This is also fragile; pthread.h might be unavailable while building
libcxx but installed later, and available when users include the
libcxx headers.

In practice, in every modern setup for building libcxx for Windows
I've seen, users end up manually configuring it with
`LIBCXX_HAS_WIN32_THREAD_API=ON`, as the users may have winpthreads
installed (for other libraries/projects to use) while wanting to
build libcxx with the default win32 threading.

Don't automatically pick up pthreads on Windows even if the header
is available. Instead require the user to configure the libcxx
build with `LIBCXX_HAS_PTHREAD_API=ON` if that's desired.

Differential Revision: https://reviews.llvm.org/D110975