use CMake's built in pthread detection

CMake ships with a standard FindThreads module which can be used to a)
test for a threading library and b) confirm that it is pthread. It also
allows the hard-coding of the threading library name to be removed from
``src/CMakeLists.txt``.

Make it an error not to have a pthread library installed and indicate to
CMake that we strongly prefer pthread to any other platform threading
library.