timegm: add portable implementation (Solaris support)

The timegm(3) function is a non-standard extension to libc which is
available in GNU libc and on some BSDs.  Although SunOS had this
function in its libc, Solaris (unfortunately) removed it.  This patch
implements a very simple version of timegm() which is good enough for
parse-time-string.c.

Signed-off-by: Vladimir Marek <vlmarek@volny.cz>