[boost] [modularization] mplcore depends on mpl

Hi there, libs/mpl/core/include/boost/mpl/aux_/include_preprocessed.hpp from mplcore includes boost/mpl/aux_/config/compiler.hpp which is in mpl. mpl depends on mplcore. This is a circular dependency. I thought the point of the split was to resolve that in part. If so, then I recommend you go ahead and resolve the cycle in the obvious way. Thanks, Steve.