Squashed 'site_scons/' changes from 0098885..909e0ee

909e0ee Use darwin and clang-darwin for gcc and clang toolsets on Darwin

git-subtree-dir: site_scons
git-subtree-split: 909e0eed65bd9c04c0cbff9f52ea77f10a308010
Merge commit '84403b7c8f6ce9f5c27a72039b9856c9e1f6565f'