update bundled threading support for GMock
Historically FindPthreads was used, but there's no good reason for that. Probably FindThreads can be used, and this would be simpler.
Fix pthread detection for GMock
FindPthreads.cmake was removed recently because it was an orphan.
It turns out that isn't true, because GMock was using it.
Rewrote that part of the build system to use the standard
Also clarified that the CMakeLists.txt is part of the Gromacs build
system instead of being part of Google Mock.