Building pthreads egg on MacOS

[Lassi Kortela]

`chicken-install pthreads` fails to compile the following C code on Mac:

void
C_pthread_pre_init(void *intres)
{
  .
  .
  .
  if(pthread_setschedprio(pthread_self(), 
sched_get_priority_max(sched_getscheduler(0)))) {
    fprintf(stderr, "Failed to raise main thread priority.\n");
  }
}

because Darwin is missing pthread_setschedprio() and 
sched_getscheduler() -- even though these are POSIX standard functions.

What should we do?

https://stackoverflow.com/questions/12056491/how-to-set-thread-priority-pthreads-on-mac-os 
says "You can only set the priority of a thread when you create it using 
pthread_setschedparam()." The current thread (pthread_self()) has 
already been created so we probably can't use pthread_setschedparam() on it.

The simplest option would be an #ifdef to comment out the above lines of 
code on Darwin.