strftime() calls tzset() unnecessarily even if no timezone information is
required. Since tzset() involves at least single stat('/etc/localtime') syscall,
that can be significant overhead for applications that call strftime() very
frequently (e.g. gbookmark GLib component, see
http://bugzilla.gnome.org/show_bug.cgi?id=535223#c4). Attached patch makes
strftime() call tzset() only when actually necessary.

It might be preferrable to ratelimit tzset() stat() calls, to say single call
per few seconds - there's no sense in checking for timezone change thousands of
times per second.