Judging from the output of brew —env, there is a MACOSX_DEPLOYMENT_TARGET you should be able to set/override. Simplest way would be when running brew:
MACOSX_DEPLOYMENT_TARGET=10.6 brew … That, in combination with —build-from-source when installing packages, might give you want you need.
That could work, but then I'm back to building from source. (macports uses binaries for most stuff, btw.)
I'm happy to investigate further _if_ a homebrew dev says that they officially support installing this way. There's a similar way to change the deployment target for macports. But one of the devs told me that kind of compatibility isn't a design goal and they don't support doing that. -Jonathan