Add Boost.Build support for the Git modular layout, using links to to new header locations, thus preventing breakage to vast amounts of user code and build setups. Being added to SVN so that changes will automatically propagate to Git and modular Boost. Thanks to Steven Watanabe, Jürgen Hunold, Vladimir Prus, and Bjørn Roald for developing this patch. Steven in particular conceived the approach and did the initial implementation, which was further refined by the other contributors.

[SVN r86392]