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

[Joseph Myers]

On Mon, 6 Jan 2025, DJ Delorie wrote:

> diff --git a/configure.ac b/configure.ac
> index 9af8d541e5..4934b044ce 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -2,6 +2,7 @@ dnl Process this file with autoconf to produce a configure script.
>  dnl Note we do not use AC_PREREQ here!  See aclocal.m4 for what we use instead.
>  AC_INIT([GNU C Library], [(see version.h)], [https://sourceware.org/bugzilla/],
>    [glibc], [https://www.gnu.org/software/glibc/])
> +AC_PREFIX_DEFAULT([/usr])

Does the AC_PREFIX_DEFAULT interface require such an autoconf-time 
constant at an early point in configure.ac?  (The proper default for Hurd 
is empty - no /usr - as used in build-many-glibcs.py.)

It looks like the previous commit regarding the default was missing a NEWS 
entry, and also an update to install.texi (+ INSTALL regeneration) to 
document the changed default.

-- 
Joseph S. Myers
josmyers@redhat.com