Since 63bd0db1 we don't use tzname
anymore, so we don't need to check for it.  Instead, just keep the
part of PGAC_STRUCT_TIMEZONE that we need, which is the check for
struct tm.tm_zone.

Discussion: https://www.postgresql.org/message-id/flat/5eb11a37-f3ca-5fb7-308f-4485dec25a2e%402ndquadrant.com