configure.ac: Set default value for the 'prefix' variable.

The Guix standard configuration uses a localstatedir of /var and a sysconfdir
of /etc.  To ease things for everyone, make the default values match that
standard expected configuration.  See
<https://lists.gnu.org/archive/html/guix-devel/2024-05/msg00003.html> for a
related discussion.

* configure.ac: Default $prefix to '' unless already set.
* doc/contributing.texi (Building from Git): Streamline doc.
* doc/guix-cookbook.texi (Guix environment via direnv): Likewise.

Reviewed-by: Ludovic Courtès <ludo@gnu.org>
Change-Id: I23cd12b58a842d246fbc9fdc740311c573eb0212

configure.ac

@@ -73,9 +73,11 @@ AC_ARG_ENABLE([daemon],
   [guix_build_daemon="$enableval"],
   [guix_build_daemon="yes"])
 
-# Prepare a version of $localstatedir & co. that does not contain references
-# to shell variables.
-guix_prefix="`eval echo $prefix | sed -e"s|NONE|/usr/local|g"`"
+dnl Prepare a version of $localstatedir & co. that does not contain references
+dnl to shell variables.  Also set some sane default directory variables for
+dnl use with the Guix.  This also causes localstatedir to be /var and
+dnl sysconfdir to be /etc.
+test "$prefix" = NONE && prefix=
 guix_localstatedir="`eval echo $localstatedir | sed -e "s|NONE|$guix_prefix|g"`"
 guix_sysconfdir="`eval echo $sysconfdir | sed -e "s|NONE|$guix_prefix|g"`"
 guix_sbindir="`eval echo $sbindir | sed -e "s|NONE|$guix_prefix|g"`"

doc/contributing.texi

@@ -235,7 +235,7 @@ more information.
 Then, run:
 
 @example
-./configure --localstatedir=/var --sysconfdir=/etc
+./configure
 @end example
 
 @noindent
@@ -246,19 +246,6 @@ normal @code{sysconfdir} value.  Note that you will probably not run
 important to pass the right @code{localstatedir} and @code{sysconfdir}
 values, which get recorded in the @code{(guix config)} Guile module.
 
-When configuring Guix on a system that already has a Guix installation,
-be sure to specify the same state directory as the existing installation
-using the @option{--localstatedir} option of the @command{configure}
-script (@pxref{Directory Variables, @code{localstatedir},, standards,
-GNU Coding Standards}).  Usually, this @var{localstatedir} option is set
-to the value @file{/var}.  The @command{configure} script protects
-against unintended misconfiguration of @var{localstatedir} so you do not
-inadvertently corrupt your store (@pxref{The Store}).  The configuration
-directory should also be configured by setting the @option{--sysconfdir}
-option to the @file{/etc} value, which is the location used by Guix to
-store for example the access control list of authorized machines and the
-definition of offload machines.
-
 Finally, you can build Guix and, if you feel so inclined, run the tests
 (@pxref{Running the Test Suite}):
 

doc/guix-cookbook.texi

@@ -5093,7 +5093,7 @@ use_guix()
     # Predefine configure flags.
     configure()
     @{
-        ./configure --localstatedir=/var --prefix=
+        ./configure
     @}
     export_function configure