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
FindThreads.cmake instead.

Also clarified that the CMakeLists.txt is part of the Gromacs build
system instead of being part of Google Mock.

Fixes #1310

Change-Id: I5055569b8534586c4faf3732d0e97c63ad7de408