On most platforms, certain compiler and linker flags have to be passed
when using pthreads, otherwise linking against libomp.so might fail with
undefined references to several pthread functions.
Use CMake's `find_package(Threads)` to determine these for standalone
builds, or take them (and optionally modify them) from the top-level
LLVM cmake files.
Also, On FreeBSD, ensure that libomp.so is linked against libm.so,
similar to NetBSD.
Adjust test cases with hardcoded `-lpthread` flag to use the common
build flags, which should now have the required pthread flags.
Reviewers: emaste, jlpeyton, krytarowski, mgorny, protze.joachim, Hahnfeld
Reviewed By: Hahnfeld
Subscribers: AndreyChurbanov, tra, EricWF, Hahnfeld, jfb, jdoerfert, openmp-commits
Tags: #openmp
Differential Revision: https://reviews.llvm.org/D59451
llvm-svn: 357618
956168c802