librsvg @2.42.2 - can't build on <10.7 due to dependecy on rust

[kencu (Ken)]

rust won't build on 10.6.8 -- there might be a version from several years ago that will build, but the last one I tried, that matches up with commit <272512b7cb6b6320a97cad307ba10fa96131f95a>, didn't build due to lack of thread_local storage. This is probably not fixable, unless I figure out a way to get thread_local into 10.6.8. Perhaps some usable version of rust older than that might build.

There are not many ports that need rust, but one of the big ones is now librsvg, upon which more than 60 ports depend.

Options are to find a version of rust that is useful that build on 10.6.8, or (more likely) to peg a version of librsvg at that previous version, that didn't require rust to build.