Draft: FindThreads: honor THREADS_PREFER_PTHREAD_FLAG when pthread is part of libc
Address the concerns raised by the recent release of glibc based systems which no longer have an independent pthread library.
Requires a change of behaviour as shown and documented in the second patch, and is using for convenience the partial fix that was presented as part of !7131 (closed) and that is independently required as a fix for AIX.
Edited by Carlo Arenas