[_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.

include/empthread.h

@@ -80,9 +80,6 @@ typedef struct lwpSem empth_sem_t;
 #endif /* _EMPTH_LWP */
 
 #ifdef _EMPTH_POSIX
-#ifdef __linux__
-#define _MIT_POSIX_THREADS 1
-#endif
 #include <pthread.h>
 #define EMPTH_FD_READ   0x1
 #define EMPTH_FD_WRITE  0x2
@@ -110,15 +107,6 @@ typedef struct {
 
 #endif /* _EMPTH_POSIX */
 
-/* DEC has slightly different names for whatever reason... */
-#ifdef _DECTHREADS_
-#define pthread_key_create  pthread_keycreate
-#define pthread_attr_init   pthread_attr_create
-#define pthread_attr_destroy pthread_attr_delete
-
-#endif
-
-
 #if defined(_EMPTH_WIN32)
 /* The Windows NT Threads */
 #define EMPTH_FD_READ   0x1