@Andrepuel docummented a nice '-lpthread' trick to get legacy
tracey::mutex() class to work on c++11. I rather like this older
implementation just because it has been deeply tested in real
production scenarios. So, reverted tracey::mutex class(). Tweaked a
couple of safety checks, warnings and lines in README.
Merge branch 'master' of https://github.com/r-lyeh/tracey into HEAD