[boost] A quieter clang build

I added the following lines to my user-config.jam: using clang-darwin : noC99W : "/usr/bin/clang++" : <cxxflags>"-Wno-c99-extensions -Wno-variadic-macros" ; and then built the 1.56.0 release with "toolset=clang-darwin-NoC99W", and lo! there were a *sane* amount of warnings. Just a quick tip for people drowning under the warnings. — Marshall P.S. I’m leaving for vacation this afternoon, but would like to pick this back up when I return.