Squashed 'site_scons/' changes from ea4558b..93bb5a6

93bb5a6 Make clang available so long as there is a default and list toolchains

git-subtree-dir: site_scons
git-subtree-split: 93bb5a6ded3810149201cd9e93f051f0e393827a
Merge commit 'bd4da381dec35308553c7e2ad09ddc3dd942671a'