use timegm() for krb5int_gmt_mktime() when available

use timegm() for krb5int_gmt_mktime() when available

Use timegm() if it is available, so that krb5int_gmt_mktime()
functions correctly on systems configured with a "right"
(leap-second-aware) time zone.  It is arguably an OS bug if a "right"
time zone can be configured on a system that lacks timegm().

Due to a current lack of evidence of affected systems, the additional
workaround of replacing gmtime() with a version that always ignores
leap seconds is deferred.

ticket: 6928

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25018 dc483132-0cff-0310-8789-dd5450dbe970