I came across a library on github (now Zlib/Libpng licensed) that is a fork of a library that was originally released under the Boost software license. The question I have is, is it legitimate/legal to relicense boost software under the zlib license? I guess the Boost license doesn't explicitly deny nor explicitly allow relicensing as far as I can tell, so I thought it was worth a question.

See: https://github.com/r-lyeh/units