Not all pthreads implementations use an integer type for pthread_t.
As a matter of fact, POSIX has explicitly withdrawn the requirement
that it must be an arithmatic type.

The musl C library uses a `struct __pthread *` which triggered the
warnings.  As of this change, sanei_thread.h works around this by
providing two new macros to help keep this issue out of sight.  All
backends have been changed to use these macros.