strerror_r() detection changes:

1. Try with _THREAD_SAFE instead of _REENTRANT, as AIX seems to require it
   and if _REENTRANT is required we should already have it set since one of
   the previous tests.
2. Added API-detection for what kind of strerror_r() that is provided. The
   POSIX style or the glibc style.

Tor Arntsen provided the necessary feedback these changes are based upon.