This has already been reported in homebrew (which as the gmp formula) and eventmachine - but don't think either sees it as a bug they can address. I'm hoping puppet-ruby is the right place to resolved this.
A few people have been reporting issues installing the eventmachine gem, and the most common reported error seems to be:
ld: library not found for -lgmp
I was one of those users and found that I could link with:
DYLD_LIBRARY_PATH="/opt/boxen/homebrew/lib"
Keeping that set permanently causes other problems, though.
I haven't had any issues linking to libraries installed via under /opt/boxen/homebrew before, so it seems odd that I would need to manually set the DYLD_LIBRARY_PATH.
What's going wrong here? Is boxen supposed to configure the system so gem's will link against /opt/boxen/homebrew/lib by default, should gmp be installing somewhere else even if homebrew --prefix is /opt/boxen/homebrew, or is there something wrong with the eventmachine extconf.rb?