configure: make --prefix=/usr and '' identical

[Andreas Schwab]

On Jan 08 2025, Joseph Myers wrote:

> Maybe having sysdeps/unix/preconfigure.ac and sysdeps/hurd/preconfigure.ac 
> that call AC_PREFIX_DEFAULT inside a suitable case condition on the host 
> OS could work?

That cannot work.  AC_PREFIX_DEFAULT adds the assignment to the DEFAULTS
diversion, which is emitted by AC_INIT at the head of the generated
configure script.  That does not work for the sysdep configure
fragments, since they are processed independently and cannot contribute
to the main configure script.  Also, AC_PREFIX_DEFAULT cannot be used
conditionally.

-- 
Andreas Schwab, schwab@linux-m68k.org
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"And now for something completely different."