Summary:
Allow using WindowsThread with Mingw

Most Mingw builds require Posix threads in order to use std::thread.
As per https://github.com/facebook/rocksdb/issues/7764, this is not always the case.

That being considered, we're going to improve the Mingw thread model
checks.

Closes: https://github.com/facebook/rocksdb/issues/7764Signed-off-by: NLucian Petrut <lpetrut@cloudbasesolutions.com>

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8108

Reviewed By: jay-zhuang

Differential Revision: D27365778

Pulled By: mrambacher

fbshipit-source-id: 2c15b1f04ae90e1e3a25a33e86ceb779224a9529