This is actually a question (or, perhaps, a bug, depending on how "it just works" you expect the system to be).
The problem, as we know if 10.4 and 10.5 is that the system provided libraries are older than what some software require, which may make, say, the configure step of some programs simply fail rejecting the versions that we have in our system of, say, libxml2 (I forget which packages I tried to install, but I saw this kind of error more than once).
With that in mind, what would be the proper/elegant way of telling a formula that, on tiger or leopard, we not only require a specific package, but also tell the build system to use that version, especially if the dependency is a keg-only formula?
I would like to fix some of these issues here and, of course, once I do that, send some ammended formulas for tigerbrew.
Most of the time, just adding depends_on 'foo' if MacOS.version <= :leopard will do the trick. depends_on does some magic to ensure that CFLAGS are set up appropriately, and in most circumstances software will pick it up without any other work.
depends_on 'foo' if MacOS.version <= :leopard
Depending on the package you might need to set a configure flag or edit the makefile, however.
Looks like this was resolved, so I'm closing this issue - but let me know if you're still having any problems with this.