Merge topic 'FindThreads-cleanup'

c53748a377 FindThreads: Factor out helper macro for libc check
564385194b FindThreads: Avoid repeating check for -pthread flag
41ef904e81 FindThreads: Simplify conditions for not-in-libc checks
cbe7550d99 FindThreads: Remove unnecessary condition
1ee8c545dd FindThreads: Clarify internal helper macro names
5b5cd92363 FindThreads: Drop strange SunOS+pthread.h+thr_create combination
c9bd462c08 FindThreads: Drop unused internal CMAKE_HAVE_THREADS_LIBRARY variable

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Acked-by: Rolf Eike Beer <eike@sf-mail.de>
Merge-request: !7207