For pthreads platforms, we have a terribly large condition for the size
of a MutexImpl that attempts to hardcode the number of words that
pthread_mutex_t takes. This hardcoding isn't always correct. We
originally did this to try and keep <pthread.h> includes out of the
headers, but the PlatformConditionVariable.h header already includes
<pthread.h> anyway, and <pthread.h> isn't so namespace-invasive as
<windows.h>. So go ahead and include <pthread.h> and use the actual
definition of pthread_mutex_t to size the platformData_ member.
Nathan Froyd