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

[Joseph Myers]

On Tue, 7 Jan 2025, DJ Delorie wrote:

> Joseph Myers <josmyers@redhat.com> writes:
> > Does the AC_PREFIX_DEFAULT interface require such an autoconf-time 
> > constant at an early point in configure.ac?
> 
> The problem is that we pull in sysdeps config files from the most
> specific to the generic, so setting --prefix in the linux config means
> it won't be set in the linux/x86_64 config, which is where we set
> libc_cv_complocaledir

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?  (Arguably the first should be 
sysdeps/unix/sysv/linux/preconfigure.ac, but then the preconfigure logic 
would need to change to look at preconfigure fragments in deeper 
subdirectories of sysdeps.)

-- 
Joseph S. Myers
josmyers@redhat.com