Squashed 'site_scons/' changes from 524ad2e..84658cc

84658cc Add the new --boost-build-once flag to minimise calls to bjam
c9d0bd9 Read configure values in as the correct type

git-subtree-dir: site_scons
git-subtree-split: 84658cc64b719ffc539eb894bc9a3b7ca39c0e4b
Merge commit '02f515b5997fdfa572053e109f0a964bd9ac8227'