Possibly even virtual/mpi...
Thanks for catching this up! Checked all implementations of virtual/mpi - building with all of them provides necessary Boost libraries. But virtual/mpi is not keyworded on ~amd64-fbsd, while Boost is. @amd64-fbsd: guys, the only MPI package in virtual/mpi keyworded on ~amd64-fbsd is sys-cluster/openmpi
+ 27 Feb 2015; Sergey Popov <pinkbyte@gentoo.org> boost-1.55.0-r2.ebuild, + boost-1.56.0-r1.ebuild, boost-1.57.0.ebuild: + Set MPI dependency to virtual/mpi, wrt bug #525596