shouldn't xz-bootstrap usage be controlled through "base"?

[RJVB (René Bertin)]

I just notice that port:libunwind extract_depends on port:xz-bootstrap, in order to minimise dependency tree.

As it turns out it does the opposite for people who build the port from source when they already have port:xz installed... If port:xz-bootstrap is indeed only used by port:clang-3.4 it's a bit silly to impose the installation on systems where that OLD compiler is devoid of (almost) all interest...

Of course each individual port could just check [file exists ${prefix}/bin/xz] to determine whether a depends_extract is required, but isn't that something that should be part of the central use_xz codepaths in "base"? AFAICT this approach should be perfectly safe without unwanted side-effects since it's relevant for building from source only.