Code review comment for ~dirk.zimoch/epics-base:epicsMutexPriorityInheritance

I tried the simply minded (early morning) test of removing the two #undef lines. It seems like to have the desired effect. pthread_mutexattr_setprotocol(..., PTHREAD_PRIO_INHERIT) succeeds with for a normal unprivleged user. I added some prints to confirm that status==0.

Am I missing something obvious?

I'm testing with Debian 10, where _XOPEN_SOURCE=700 (by way of -D_GNU_SOURCE).

Also, what is the motivation to economize pthread_mutexattr_t? I see __SIZEOF_PTHREAD_MUTEXATTR_T==4 (on a 64-bit host).