Ignore errors from rm -rf to support high -j builds. This is, at best,
a kludge. However, it also effectively works around the issues for
high -j builds on systems that do not have the rm fixes.

A better fix would be to rmdir here, and fix the places where we're
sloppy and not list all the files we create in CLEANFILES, should
anybody have the time to chase them all to ground.