Fix: timegm compat on Solaris

On Solaris 11 setting TZ to an empty string before calling mktime will
in some circumstances do the conversion in localtime instead of UTC as
expected.

Replace the empty string by "UTC".

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>