New default version for librsvg2.

  Current versions of graphics/librsvg2 are using parts written in rust.
  For architectures that do not have support for rust, and for people
  who prefer not to have, or are not able to compile rust software due
  to hardware limitations, the version can be chosen via this new flag.

  The default on almost all architectures is rust.

  If you prefer no to use rust, add the following to your make.conf:

        DEFAULT_VERSIONS+=librsvg2=legacy

This makes the change committed in r554733 a bit more flexible for people
who are opposed to oxidation.

PR:		251014
Exp-run by:	antoine