This uses SRWLock on Vista and above, and uses a hand-rolled implementation on Windows XP that uses Events (and a spinlock, if this is a multi-core system).
Since SRWLocks aren't recursive, ReMutexWin32Impl has been added to implement recursive mutices, using old-fashioned critical sections.
MutexImpl now has a constexpr constructor on all implementations.
This renames acquire/release to lock/unlock in order to be compatible with std::lock_guard and std::unique_lock (which will eventually replace the *MutexHolder classes). It will also allow us to typedef MutexImpl to std::mutex later on.
rdb
tobspr
David Rose