POSIX and your local friendly ctime(3) manual entry says that localtime_r isn't
required to set the state variables that define the current timezone. Indeed,
glibc (at least 2.9) doesn't do this for subsequent calls. The effect is that
if the system timezone is changed for a running program between two calls to
gst_clock_overlay_render_time, it won't be noticed. For glibc, changing the
timezone equals /etc/localtime being modified.
Fixes bug #587676.
04e23f2d6a