[boost] [build] Overriding warning level for clang

In my user-config.jam for clang I have added: <warnings>on to the configuration section of the 'using clang' toolset. Nonetheless I still see '-Wall' on the command line when I compile. My other <cxxflags> settings which are part of the toolset are reflected by the command line. Even if I change to <warnings>off in the toolset I still see '-Wall' on the command line. Why is this not working and is there any way I can change the warning level for clang so that it is not 'Wall' on the command line ? The clang compiler is wonderful but with 'Wall' it spews out a boatload of information I normally do not need.