Test for hardware support for atomics, link to libatomic if not detected

[barracuda156]

@ConorWilliams Will this work? On ppc it works correctly, test fails and -latomic is added to linker flags:

/opt/local/bin/g++-mp-14 -pipe -Os -DNDEBUG -I/opt/local/include -D_GLIBCXX_USE_CXX11_ABI=0 -arch ppc -mmacosx-version-min=10.6 -Wl,-search_paths_first -Wl,-headerpad_max_install_names -L/opt/local/lib -Wl,-headerpad_max_install_names CMakeFiles/libfork_test.dir/source/algorithm/explicit.cpp.o CMakeFiles/libfork_test.dir/source/algorithm/fold.cpp.o CMakeFiles/libfork_test.dir/source/algorithm/for_each.cpp.o CMakeFiles/libfork_test.dir/source/algorithm/map.cpp.o CMakeFiles/libfork_test.dir/source/algorithm/scan.cpp.o CMakeFiles/libfork_test.dir/source/catch.cpp.o CMakeFiles/libfork_test.dir/source/core/bad_behavior.cpp.o CMakeFiles/libfork_test.dir/source/core/core.cpp.o CMakeFiles/libfork_test.dir/source/core/deque.cpp.o CMakeFiles/libfork_test.dir/source/core/eventually.cpp.o CMakeFiles/libfork_test.dir/source/core/exceptions.cpp.o CMakeFiles/libfork_test.dir/source/core/explicit.cpp.o CMakeFiles/libfork_test.dir/source/core/extern.cpp.o CMakeFiles/libfork_test.dir/source/core/extern_test.cpp.o CMakeFiles/libfork_test.dir/source/core/just.cpp.o CMakeFiles/libfork_test.dir/source/core/lazy_pool.cpp.o CMakeFiles/libfork_test.dir/source/core/result.cpp.o CMakeFiles/libfork_test.dir/source/core/unbalanced.cpp.o CMakeFiles/libfork_test.dir/source/core/wsq.cpp.o CMakeFiles/libfork_test.dir/source/macro.cpp.o CMakeFiles/libfork_test.dir/source/numa.cpp.o CMakeFiles/libfork_test.dir/source/schedule/event_count.cpp.o -o libfork_test  -Wl,-rpath,/opt/local/lib /opt/local/lib/libCatch2Main.a -latomic /opt/local/lib/libCatch2.a

P. S. 1-byte atomics may be needed by RISCV, so I added that too.

[ConorWilliams]

Why only for GNU?

[barracuda156]

It is a gcc library. It won’t exist without gcc and should not be used by something other than gcc.

(I think clang uses compiler-rt, but not sure and cannot test that.)