[boost] Building with clang for Windows/MingW

When trying to test clang under Windows which is built with MingW, Boost Build runs with the options: -c -x c++ -O0 -g -fno-inline -Wall -g -mthreads However clang does not recognize -mthreads and flags it as an error. Is there anything I can do to remove this option from the clang++.exe command line when invoking Boost Build ?