gnu: glibc: Honor 'GUIX_LOCPATH'.

* gnu/packages/patches/glibc-guix-locpath.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/base.scm (glibc)[source]: Use it.
  [native-search-paths]: Use 'GUIX_LOCPATH' instead of 'LOCPATH'.
* doc/guix.texi (Application Setup): Introduce the term "foreign
  distro".  Document 'GUIX_LOCPATH'.

doc/guix.texi

@@ -933,24 +933,24 @@ daemons on the same machine.
 @node Application Setup
 @section Application Setup
 
-When using Guix on top of GNU/Linux distribution other than GuixSD, a
-few additional steps are needed to get everything in place.  Here are
-some of them.
+When using Guix on top of GNU/Linux distribution other than GuixSD---a
+so-called @dfn{foreign distro}---a few additional steps are needed to
+get everything in place.  Here are some of them.
 
 @subsection Locales
 
 @anchor{locales-and-locpath}
 @cindex locales, when not on GuixSD
 @vindex LOCPATH
+@vindex GUIX_LOCPATH
 Packages installed @i{via} Guix will not use the host system's locale
 data.  Instead, you must first install one of the locale packages
-available with Guix and then define the @code{LOCPATH} environment
-variable (@pxref{Locale Names, @code{LOCPATH},, libc, The GNU C Library
-Reference Manual}):
+available with Guix and then define the @code{GUIX_LOCPATH} environment
+variable:
 
 @example
 $ guix package -i glibc-locales
-$ export LOCPATH=$HOME/.guix-profile/lib/locale
+$ export GUIX_LOCPATH=$HOME/.guix-profile/lib/locale
 @end example
 
 Note that the @code{glibc-locales} package contains data for all the
@@ -958,6 +958,18 @@ locales supported by the GNU@tie{}libc and weighs in at around
 110@tie{}MiB.  Alternately, the @code{glibc-utf8-locales} is smaller but
 limited to a few UTF-8 locales.
 
+The @code{GUIX_LOCPATH} variable plays the exact same role as
+@code{LOCPATH} (@pxref{Locale Names, @code{LOCPATH},, libc, The GNU C
+Library Reference Manual}).  However, since it is honored only by Guix's
+libc, and not by the libc provided by foreign distros, using
+@code{GUIX_LOCPATH} allows you to make sure the the foreign distro's
+programs will not end up loading incompatible locale data.  This is
+important because the locale data format used by different libc versions
+may be incompatible.
+
+When both @code{GUIX_LOCPATH} and @code{LOCPATH} are defined, the latter
+takes precedence.
+
 @subsection X11 Fonts
 
 The majority of graphical applications use Fontconfig to locate and

gnu-system.am

@@ -468,6 +468,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/glib-tests-timer.patch			\
   gnu/packages/patches/glib-tests-gapplication.patch		\
   gnu/packages/patches/glibc-bootstrap-system.patch		\
+  gnu/packages/patches/glibc-guix-locpath.patch			\
   gnu/packages/patches/glibc-ldd-x86_64.patch			\
   gnu/packages/patches/glibc-locales.patch			\
   gnu/packages/patches/glibc-locale-incompatibility.patch	\

gnu/packages/base.scm

@@ -484,6 +484,7 @@ (define-public glibc
             (patches (map search-patch
                           '("glibc-ldd-x86_64.patch"
                             "glibc-locale-incompatibility.patch"
+                            "glibc-guix-locpath.patch"
                             "glibc-o-largefile.patch")))))
    (build-system gnu-build-system)
 
@@ -614,9 +615,11 @@ (define-public glibc
 
    (native-search-paths
     ;; Search path for packages that provide locale data.  This is useful
-    ;; primarily in build environments.
+    ;; primarily in build environments.  Use 'GUIX_LOCPATH' rather than
+    ;; 'LOCPATH' to avoid interference with the host system's libc on foreign
+    ;; distros.
     (list (search-path-specification
-           (variable "LOCPATH")
+           (variable "GUIX_LOCPATH")
            (files '("lib/locale")))))
 
    (synopsis "The GNU C Library")

gnu/packages/patches/glibc-guix-locpath.patch

@@ -0,0 +1,34 @@
+Honor a special 'GUIX_LOCPATH' environment variable.
+
+This is most useful when using Guix on top of another distro, which uses an
+different libc version with incompatible locale data.  In this case, setting
+'GUIX_LOCPATH' rather than 'LOCPATH' allows users to tell Guix's libc where to
+look for its locale data without breaking programs that use the other libc.
+
+See <https://lists.gnu.org/archive/html/guix-devel/2015-09/msg00717.html> for
+some background information.
+
+--- a/locale/newlocale.c
++++ b/locale/newlocale.c
+@@ -103,6 +103,8 @@ __newlocale (int category_mask, const char *locale, __locale_t base)
+   locale_path_len = 0;
+ 
+   locpath_var = getenv ("LOCPATH");
++  if (locpath_var == NULL || locpath_var[0] == '\0')
++    locpath_var = getenv ("GUIX_LOCPATH");
+   if (locpath_var != NULL && locpath_var[0] != '\0')
+     {
+       if (__argz_create_sep (locpath_var, ':',
+diff --git a/locale/setlocale.c b/locale/setlocale.c
+index ead030d..0e66c7b 100644
+--- a/locale/setlocale.c
++++ b/locale/setlocale.c
+@@ -252,6 +252,8 @@ setlocale (int category, const char *locale)
+   locale_path_len = 0;
+ 
+   locpath_var = getenv ("LOCPATH");
++  if (locpath_var == NULL || locpath_var[0] == '\0')
++    locpath_var = getenv ("GUIX_LOCPATH");
+   if (locpath_var != NULL && locpath_var[0] != '\0')
+     {
+       if (__argz_create_sep (locpath_var, ':',