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+dnl Reentrant time functions: localtime_r, gmtime_r.
+
+dnl Copyright (C) 2003, 2006-2018 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.
+
+dnl Written by Paul Eggert.
+
+AC_DEFUN([gl_TIME_R],
+[
+  dnl Persuade glibc and Solaris <time.h> to declare localtime_r.
+  AC_REQUIRE([gl_USE_SYSTEM_EXTENSIONS])
+
+  AC_REQUIRE([gl_HEADER_TIME_H_DEFAULTS])
+  AC_REQUIRE([AC_C_RESTRICT])
+
+  dnl Some systems don't declare localtime_r() and gmtime_r() if _REENTRANT is
+  dnl not defined.
+  AC_CHECK_DECLS([localtime_r], [], [], [[#include <time.h>]])
+  if test $ac_cv_have_decl_localtime_r = no; then
+    HAVE_DECL_LOCALTIME_R=0
+  fi
+
+  AC_CHECK_FUNCS_ONCE([localtime_r])
+  if test $ac_cv_func_localtime_r = yes; then
+    HAVE_LOCALTIME_R=1
+    AC_CACHE_CHECK([whether localtime_r is compatible with its POSIX signature],
+      [gl_cv_time_r_posix],
+      [AC_COMPILE_IFELSE(
+         [AC_LANG_PROGRAM(
+            [[#include <time.h>]],
+            [[/* We don't need to append 'restrict's to the argument types,
+                 even though the POSIX signature has the 'restrict's,
+                 since C99 says they can't affect type compatibility.  */
+              struct tm * (*ptr) (time_t const *, struct tm *) = localtime_r;
+              if (ptr) return 0;
+              /* Check the return type is a pointer.
+                 On HP-UX 10 it is 'int'.  */
+              *localtime_r (0, 0);]])
+         ],
+         [gl_cv_time_r_posix=yes],
+         [gl_cv_time_r_posix=no])
+      ])
+    if test $gl_cv_time_r_posix = yes; then
+      REPLACE_LOCALTIME_R=0
+    else
+      REPLACE_LOCALTIME_R=1
+    fi
+  else
+    HAVE_LOCALTIME_R=0
+  fi
+])
+
+# Prerequisites of lib/time_r.c.
+AC_DEFUN([gl_PREREQ_TIME_R], [
+  :
+])