boost156: remove separate lib directory

This is more of a question than a PR -- why do we have separate -lib output for Boost 1.56? I was stuck for some time wondering where had /lib disappeared. It also is not included into LIBRARY_PATH, so my package for which I've tried to use it failed to link. (with this patch it works) Also -- how can I access this separate -lib output from another package?

CC @peti @wkennington (maintainers) Thanks for the answers!