this is documented in doc/install.texi:

"""
This option is only useful when you are already using @option{--with-sysroot}."""

now, lets error out if you configure --with-build-sysroot without
--with-sysroot. Ok for the trunk?

This was hit in PR55743. I think the patch in
http://gcc.gnu.org/ml/gcc-patches/2013-01/msg00198.html is still valid, but
independent of this.

  Matthias