PR 32272: Error out if building separately but srcdir has build remnants.

[DJ Delorie]

> If the user built in-tree at some point in time, and things failed to
> clean up right,

"Failed to clean up right"?  Can we get a confirmation that "make
clean" or "make distclean" *does* clean up right?

Also, if we've configured in the source tree, there would be other
signs - config.log, Makefile, etc.  We should check for all those
things (and this one) at the same time.  If you're going to be a
little paranoid, might as well be a lot paranoid.