[Bug libstdc++/59177] steady_clock::now() and system_clock::now do not use the vdso (and are therefore very slow)

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59177

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The default for --enable-libstdcxx-time is =auto, which should be the right
thing:
      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


My trunk build on Fedora 19 today has:
/* Defined if clock_gettime syscall has monotonic and realtime clock support.
   */
/* #undef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL */

/* Defined if clock_gettime has monotonic clock support. */
#define _GLIBCXX_USE_CLOCK_MONOTONIC 1

/* Defined if clock_gettime has realtime clock support. */
#define _GLIBCXX_USE_CLOCK_REALTIME 1
and there are two clock_gettime calls in libstdc++.so.6 as they should be.