Change comparisons of tm->tm_isdst from "nonzero" to "greater than zero".

Change comparisons of tm->tm_isdst from "nonzero" to "greater than zero".
 Not sure why some were this way, and others were already correct, but it
 seems to have been like this for several years.
This caused problems on a few damaged platforms like AIX and IRIX which do
 not support DST calculations for years before 1970.
Thanks to Andreas Zeugswetter <ZeugswetterA@wien.spardat.at> for finding
 the problem.