(Virtual) GNU R package to provide BH

 The CRAN package BH provides a (large) subset of Boost. This package tricks
 R into believing BH is installed when we just depend on the distribution's
 Boost packages. The actual set of Boost libraries could get fine-tuned. In
 short, we avoid doubling up the 140+ mb of the 'BH' package which are alredy
 in libboost-dev.