pthread: try to avoid explicit inclusion of headers

[mojca]

Geant4 recently introduced support for multiple threads. It would make sense to check whether one can avoid reinventing the wheel and use the code from Geant4.

I have found this:

• http://web.cs.du.edu/~sturtevant/w13-sys/InstallingpthreadsforVisualStudio.pdf so in principle pthread can work, but it would be nice if the code would work out of the box even without the presence of that library.