[PATCH 1/7] Fix __PTHREAD_MUTEX_HAVE_ELISION -Wundef warning

[Roland McGrath]

> For 788bba368c2eaf8aa3fd2ca18d269395d6bc8afb I still think the ifdef for
> __PTHREAD_MUTEX_HAVE_ELISION is the way to go, since it fallows the
> __PTHREAD_MUTEX_HAVE_PREV logic already in place and avoid adds another
> default header or add more arch specific logic for arch that do not 
> support it.

Someone should audit all the related #if logic and describe what it's
doing.  Then we can consider how best to rework it to be typo-proof.