Minimal support for multilib builds.

For a little more information, see:
<http://airs.com/ian/configure/configure_8.html>

The master (and probably more up-to-date) copies of the 'config-ml.in'
and 'symlink-tree' files are maintained in the GCC development tree
at <https://gcc.gnu.org/svn.html>.  The same is probably true also for
the 'multi.m4' file.