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.