gnu: libc: Look for locale data under /run/current-system/locale/X.Y.

* gnu/packages/base.scm (glibc)[arguments]: Append VERSION to
  'libc_cv_localedir'.
* gnu/system/locale.scm (localedef-command): Write to the sub-directory
  called (package-version libc) in #$output.
  (locale-directory): Create said directory.
* doc/guix.texi (Locales): Mention the per-version sub-directory.

doc/guix.texi

@@ -5591,9 +5591,10 @@ list only the locales that are actually used, as in:
 
 @vindex LOCPATH
 The compiled locale definitions are available at
-@file{/run/current-system/locale}, which is the default location where
+@file{/run/current-system/locale/X.Y}, where @code{X.Y} is the libc
-the GNU@tie{}libc looks for locale data.  This can be overridden using
+version, which is the default location where the GNU@tie{}libc provided
-the @code{LOCPATH} environment variable (@pxref{locales-and-locpath,
+by Guix looks for locale data.  This can be overridden using the
+@code{LOCPATH} environment variable (@pxref{locales-and-locpath,
 @code{LOCPATH} and locale packages}).
 
 The @code{locale-definition} form is provided by the @code{(gnu system

gnu/packages/base.scm

@@ -509,12 +509,16 @@ store.")
             ;; Set the default locale path.  In practice, $LOCPATH may be
             ;; defined to point whatever locales users want.  However, setuid
             ;; binaries don't honor $LOCPATH, so they'll instead look into
-            ;; $libc_cv_localedir; we choose /run/current-system/locale, with
+            ;; $libc_cv_localedir; we choose /run/current-system/locale/X.Y,
-            ;; the idea that it is going to be populated by the sysadmin.
+            ;; with the idea that it is going to be populated by the sysadmin.
+            ;; The "X.Y" sub-directory is because locale data formats are
+            ;; incompatible across libc versions; see
+            ;; <https://lists.gnu.org/archive/html/guix-devel/2015-08/msg00737.html>.
             ;;
             ;; `--localedir' is not honored, so work around it.
             ;; See <http://sourceware.org/ml/libc-alpha/2013-03/msg00093.html>.
-            (string-append "libc_cv_localedir=/run/current-system/locale")
+            (string-append "libc_cv_localedir=/run/current-system/locale/"
+                           ,version)
 
             (string-append "--with-headers="
                            (assoc-ref %build-inputs "linux-headers")

gnu/system/locale.scm

@@ -19,6 +19,7 @@
 (define-module (gnu system locale)
   #:use-module (guix gexp)
   #:use-module (guix records)
+  #:use-module (guix packages)
   #:use-module (gnu packages base)
   #:use-module (gnu packages compression)
   #:use-module (srfi srfi-26)
@@ -57,14 +58,20 @@
                       "-i" #$(locale-definition-source locale)
                       "-f" #$(locale-definition-charset locale)
                       (string-append #$output "/"
+                                     #$(package-version libc) "/"
                                      #$(locale-definition-name locale))))))
 
 (define* (locale-directory locales
                            #:key (libc (canonical-package glibc)))
-  "Return a directory containing all of LOCALES compiled."
+  "Return a directory containing all of LOCALES for LIBC compiled.
+
+Because locale data formats are incompatible when switching from one libc to
+another, locale data is put in a sub-directory named after the 'version' field
+of LIBC."
   (define build
     #~(begin
         (mkdir #$output)
+        (mkdir (string-append #$output "/" #$(package-version libc)))
 
         ;; 'localedef' executes 'gzip' to access compressed locale sources.
         (setenv "PATH" (string-append #$gzip "/bin"))