(toplevel) Fix dramatic breakage for ordinary crosses (related to program_transform_name)

[DJ Delorie]

> Now you're speaking my language.  This even lets me say "make
> configure-gdb" to configure exactly the directories I need.  Sure, some
> reconfigures may end up taking place in parallel; I posit that if we
> see a config.cache problem then it should be solved by fixing
> _autoconf_ to update the cache atomically.

Not just update, but *merge*.  Autoconf reads the cache, does lots of
processing, then writes the cache.  The cache may have been updated
during the processing, and you don't want to toss those updates.

But it would be nice if autoconf managed this problem for us.  It
would mean migrating to a new version of autoconf, of course.