rtems: Avoid copy and paste in confdefs.h

Add and use CONFIGURE_POSIX_MUTEXES.
Add and use CONFIGURE_POSIX_CONDITION_VARIABLES.
libblock: Add RTEMS_BDBUF_USE_PTHREAD

Use the PTHREAD mutexes and condition variables if available.  This
helps on SMP configurations to avoid the home grown condition variables
via disabled preemption.