Use Gnulib's `duplocale' module.

* libguile/i18n.c (scm_make_locale): Simplify global locale handling,
  using duplocale(3) for all kinds of locales.
  (scm_init_i18n): Comment on why we don't just use `LC_GLOBAL_LOCALE'
  for `global_locale_smob'.

* m4/gnulib-cache.m4: Add `duplocale'.

m4/duplocale.m4

@@ -0,0 +1,56 @@
+# duplocale.m4 serial 1
+dnl Copyright (C) 2009 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_DUPLOCALE],
+[
+  AC_REQUIRE([gl_LOCALE_H_DEFAULTS])
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  AC_CHECK_FUNCS_ONCE([duplocale])
+  if test $ac_cv_func_duplocale = yes; then
+    dnl Check against glibc bug where duplocale crashes.
+    dnl See <http://sourceware.org/bugzilla/show_bug.cgi?id=10969>.
+    AC_REQUIRE([gl_LOCALE_H])
+    AC_CACHE_CHECK([whether duplocale(LC_GLOBAL_LOCALE) works],
+      [gl_cv_func_duplocale_works],
+      [AC_TRY_RUN([
+#include <locale.h>
+#if HAVE_XLOCALE_H
+# include <xlocale.h>
+#endif
+int main ()
+{
+  (void) duplocale (LC_GLOBAL_LOCALE);
+  return 0;
+}], [gl_cv_func_duplocale_works=yes], [gl_cv_func_duplocale_works=no],
+         [dnl Guess it works except on glibc < 2.12.
+          AC_EGREP_CPP([Unlucky GNU user], [
+#include <features.h>
+#ifdef __GNU_LIBRARY__
+ #if (__GLIBC__ == 2 && __GLIBC_MINOR__ < 12)
+  Unlucky GNU user
+ #endif
+#endif
+            ],
+            [gl_cv_func_duplocale_works="guessing no"],
+            [gl_cv_func_duplocale_works="guessing yes"])
+         ])
+      ])
+    case "$gl_cv_func_duplocale_works" in
+      *no) REPLACE_DUPLOCALE=1 ;;
+    esac
+  fi
+  if test $REPLACE_DUPLOCALE = 1; then
+    gl_REPLACE_LOCALE_H
+    AC_LIBOBJ([duplocale])
+    gl_PREREQ_DUPLOCALE
+  fi
+])
+
+# Prerequisites of lib/duplocale.c.
+AC_DEFUN([gl_PREREQ_DUPLOCALE],
+[
+  :
+])

m4/gnulib-cache.m4

@@ -15,7 +15,7 @@
 
 
 # Specification in the form of a command-line invocation:
-#   gnulib-tool --import --dir=. --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --lgpl=3 --libtool --macro-prefix=gl --no-vc-files alignof alloca-opt announce-gen autobuild byteswap canonicalize-lgpl environ extensions flock fpieee full-read full-write gendocs gitlog-to-changelog gnu-web-doc-update gnupload havelib iconv_open-utf inet_ntop inet_pton lib-symbol-versions lib-symbol-visibility libunistring locale maintainer-makefile putenv stdlib strcase strftime striconveh string verify version-etc-fsf vsnprintf warnings
+#   gnulib-tool --import --dir=. --lib=libgnu --source-base=lib --m4-base=m4 --doc-base=doc --tests-base=tests --aux-dir=build-aux --lgpl=3 --libtool --macro-prefix=gl --no-vc-files alignof alloca-opt announce-gen autobuild byteswap canonicalize-lgpl duplocale environ extensions flock fpieee full-read full-write gendocs gitlog-to-changelog gnu-web-doc-update gnupload havelib iconv_open-utf inet_ntop inet_pton lib-symbol-versions lib-symbol-visibility libunistring locale maintainer-makefile putenv stdlib strcase strftime striconveh string verify version-etc-fsf vsnprintf warnings
 
 # Specification in the form of a few gnulib-tool.m4 macro invocations:
 gl_LOCAL_DIR([])
@@ -26,6 +26,7 @@ gl_MODULES([
   autobuild
   byteswap
   canonicalize-lgpl
+  duplocale
   environ
   extensions
   flock

m4/gnulib-comp.m4

@@ -57,6 +57,8 @@ AC_DEFUN([gl_INIT],
   gl_MODULE_INDICATOR([canonicalize-lgpl])
   gl_STDLIB_MODULE_INDICATOR([canonicalize_file_name])
   gl_STDLIB_MODULE_INDICATOR([realpath])
+  gl_FUNC_DUPLOCALE
+  gl_LOCALE_MODULE_INDICATOR([duplocale])
   gl_ENVIRON
   gl_UNISTD_MODULE_INDICATOR([environ])
   gl_HEADER_ERRNO_H
@@ -306,6 +308,7 @@ AC_DEFUN([gl_FILE_LIST], [
   lib/c-strncasecmp.c
   lib/canonicalize-lgpl.c
   lib/config.charset
+  lib/duplocale.c
   lib/errno.in.h
   lib/float+.h
   lib/float.in.h
@@ -407,6 +410,7 @@ AC_DEFUN([gl_FILE_LIST], [
   m4/codeset.m4
   m4/dos.m4
   m4/double-slash-root.m4
+  m4/duplocale.m4
   m4/eealloc.m4
   m4/environ.m4
   m4/errno_h.m4