commit 47e011c33d78a00015265be39f2845209eeb97fc
Author: Jonathan Wakely <jwakely.gcc@gmail.com>
Date:   Sun Feb 3 20:45:51 2013 +0000

    	* acinclude.m4 (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Add KIND=auto to
    	enable features if target OS is known to support them.
    	* configure.ac (GLIBCXX_ENABLE_LIBSTDCXX_TIME): Default to 'auto'.
    	* configure: Regenerate.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 619fff0..efeb6d4 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -871,7 +871,8 @@ dnl    (FEATURE, DEFAULT, HELP-ARG, HELP-STRING)
 dnl    (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, permit a|b|c)
 dnl    (FEATURE, DEFAULT, HELP-ARG, HELP-STRING, SHELL-CODE-HANDLER)
 dnl
-dnl See docs/html/17_intro/configury.html#enable for documentation.
+dnl See manual/appendix_porting.html#appendix.porting.build_hacking for
+dnl documentation.
 dnl
 m4_define([GLIBCXX_ENABLE],[dnl
 m4_define([_g_switch],[--enable-$1])dnl
@@ -1161,8 +1162,9 @@ dnl        nanosleep and sched_yield in libc and libposix4 and, if needed,
 dnl        links in the latter.
 dnl --enable-libstdcxx-time=rt
 dnl        also searches (and, if needed, links) librt.  Note that this is
-dnl        not always desirable because, in glibc, for example, in turn it
-dnl        triggers the linking of libpthread too, which activates locking,
+dnl        not always desirable because, in glibc 2.16 and earlier, for
+dnl        example, in turn it triggers the linking of libpthread too,
+dnl        which activates locking,
 dnl        a large overhead for single-thread programs.
 dnl --enable-libstdcxx-time=no
 dnl --disable-libstdcxx-time
@@ -1175,8 +1177,7 @@ dnl os_defines.h and also defines _GLIBCXX_USE_SCHED_YIELD.
 dnl
 AC_DEFUN([GLIBCXX_ENABLE_LIBSTDCXX_TIME], [
 
-  AC_MSG_CHECKING([for clock_gettime, nanosleep and sched_yield])
-  GLIBCXX_ENABLE(libstdcxx-time,$1,[[[=KIND]]],
+  GLIBCXX_ENABLE(libstdcxx-time,auto,[[[=KIND]]],
     [use KIND for check type],
     [permit yes|no|rt])
 
@@ -1188,9 +1189,59 @@ AC_DEFUN([GLIBCXX_ENABLE_LIBSTDCXX_TIME], [
 
   ac_has_clock_monotonic=no
   ac_has_clock_realtime=no
-  AC_MSG_RESULT($enable_libstdcxx_time)
+  ac_has_nanosleep=no
+  ac_has_sched_yield=no
+
+  if test x"$enable_libstdcxx_time" = x"auto"; then
+
+    case "${target_os}" in
+      cygwin*)
+        ac_has_nanosleep=yes
+        ;;
+      darwin*)
+        ac_has_nanosleep=yes
+        ac_has_sched_yield=yes
+        ;;
+      gnu* | linux* | kfreebsd*-gnu | knetbsd*-gnu)
+        AC_MSG_CHECKING([for at least GNU libc 2.17])
+        AC_TRY_COMPILE(
+          [#include <features.h>],
+          [
+          #if ! __GLIBC_PREREQ(2, 17)
+          #error 
+          #endif
+          ],
+          [glibcxx_glibc217=yes], [glibcxx_glibc217=no])
+        AC_MSG_RESULT($glibcxx_glibc217)
+
+        if test x"$glibcxx_glibc217" = x"yes"; then
+          ac_has_clock_monotonic=yes
+          ac_has_clock_realtime=yes
+        fi
+        ac_has_nanosleep=yes
+        ac_has_sched_yield=yes
+        ;;
+      freebsd*|netbsd*)
+        ac_has_clock_monotonic=yes
+        ac_has_clock_realtime=yes
+        ac_has_nanosleep=yes
+        ac_has_sched_yield=yes
+        ;;
+      openbsd*)
+        ac_has_clock_monotonic=yes
+        ac_has_clock_realtime=yes
+        ac_has_nanosleep=yes
+        ;;
+      solaris*)
+        GLIBCXX_LIBS="$GLIBCXX_LIBS -lrt"
+        ac_has_clock_monotonic=yes
+        ac_has_clock_realtime=yes
+        ac_has_nanosleep=yes
+        ac_has_sched_yield=yes
+        ;;
+    esac
 
-  if test x"$enable_libstdcxx_time" != x"no"; then
+  elif test x"$enable_libstdcxx_time" != x"no"; then
 
     if test x"$enable_libstdcxx_time" = x"rt"; then
       AC_SEARCH_LIBS(clock_gettime, [rt posix4])
@@ -1214,19 +1265,16 @@ AC_DEFUN([GLIBCXX_ENABLE_LIBSTDCXX_TIME], [
     case "$ac_cv_search_sched_yield" in
       -lposix4*)
       GLIBCXX_LIBS="$GLIBCXX_LIBS $ac_cv_search_sched_yield"
-      AC_DEFINE(_GLIBCXX_USE_SCHED_YIELD, 1,
-		[ Defined if sched_yield is available. ])
+      ac_has_sched_yield=yes
       ;;
       -lrt*)
       if test x"$enable_libstdcxx_time" = x"rt"; then
 	GLIBCXX_LIBS="$GLIBCXX_LIBS $ac_cv_search_sched_yield"
-	AC_DEFINE(_GLIBCXX_USE_SCHED_YIELD, 1,
-		  [ Defined if sched_yield is available. ])
+        ac_has_sched_yield=yes
       fi
       ;;
       *)
-      AC_DEFINE(_GLIBCXX_USE_SCHED_YIELD, 1,
-		[ Defined if sched_yield is available. ])
+      ac_has_sched_yield=yes
       ;;
     esac
 
@@ -1284,6 +1332,11 @@ AC_DEFUN([GLIBCXX_ENABLE_LIBSTDCXX_TIME], [
       [ Defined if clock_gettime has realtime clock support. ])
   fi
 
+  if test x"$ac_has_sched_yield" = x"yes"; then
+    AC_DEFINE(_GLIBCXX_USE_SCHED_YIELD, 1,
+              [ Defined if sched_yield is available. ])
+  fi
+
   if test x"$ac_has_nanosleep" = x"yes"; then
     AC_DEFINE(_GLIBCXX_USE_NANOSLEEP, 1,
       [ Defined if nanosleep is available. ])
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index 73d430a..f6ba176 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -198,8 +198,7 @@ GLIBCXX_COMPUTE_STDIO_INTEGER_CONSTANTS
 GLIBCXX_CHECK_GETTIMEOFDAY
 
 # For clock_gettime, nanosleep and sched_yield support.
-# NB: The default is [no], because otherwise it requires linking.
-GLIBCXX_ENABLE_LIBSTDCXX_TIME([no])
+GLIBCXX_ENABLE_LIBSTDCXX_TIME
 
 AC_LC_MESSAGES