[_DECTHREADS_] Drop support for DECthreads d4, a.k.a. DCE threads,

[_DECTHREADS_] Drop support for DECthreads d4, a.k.a. DCE threads,
which are obsolete since at least 1997.

(empth_terminate, empth_wakeup) [__ppc__]: Ancient versions of OS X
lacked pthread_kill().  As a work-around, its use was disabled here
for all versions of OS X.  The work-around could lead to hangs.
Remove it and drop support for ancient versions of OS X.

(empth_create) [__linux__]: Linux has supported thread attribute
stacksize for ages.  Remove the special case.

(_MIT_POSIX_THREADS): Unused, remove.