* we've moved to cmake 3.5, so no need to hack for newer cmake
* nss dependency is dropped, no need to install it anymore
* pass clang and clang++ using cmake variable to be more consistent
* pass CMAKE_EXE_LINKER_FLAGS as `/usr/local/opt/llvm/lib` is
not in default library paths.
* mention https://github.com/boostorg/atomic/issues/15
Signed-off-by: Kefu Chai <kchai@redhat.com>
