try:
b2 clean
b2 toolset=clang cxxflags="-std=c++1y -stdlib=libc++" linkflags="-stdlib=libc++" --prefix=c:\boost -j 4 stage release
b2 install toolset=clang cxxflags="-std=c++1y -stdlib=libc++" linkflags="-stdlib=libc++" --prefix=c:\boost