Don't use macro _XOPEN_SOURCE for function strptime
We detect all necessary feature macros in configure script
using AC_USE_SYSTEM_EXTENSIONS or AC_GNU_SOURCE.
This patch replaces all definitions of macro _XOPEN_SOURCE with
header file config.h
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit 5389b3714be747f1a11ac51beb0c5988cfb6c240)