Fix build on Xcode 4.6.3 for Boost versions 1.50.0, 1.51.0, and 1.52.0. 1.53.0 (and above, I think) don't work. Be permissive with which versions of Boost are "supported" by the script and don't artificially restrict it. Next work will be fixing the build for Boost version 1.53.0-1.55.0.