Use protobuf-c when compiling osm2pgsql if installed

Despite having `protobuf-c` installed via homebrew, the osm2pgsql formula wasn't finding it unless I manually passed in configure options, even with `depends_on "protobuf-c" => :optional` already being present in the formula.

This PR fixes that, but it's not quite right. I added a configure option that passes protobuf-c `opt_prefix` into a configure option, which will still run happily even if protobuf-c isn't actually installed. Problem is that you can no longer overwrite this option via `brew install`. Let me know the homebrew idiom for handling this and I'll update my PR.