Support turning off the timezone feature

Current date-time handling is rather limited without QT_CONFIG(timezone): it is able to support Qt::LocalTime time-spec only and Qt::UTC/Qt::OffsetFromUTC are not handled correctly together.

More specifically, it happens in cases when date-time format string includes a `t` specifier and the date-time string has corresponding time zone specifier which is a UTC-offset zone identifier e.g. 2001-09-15T09:33:01.001UTC-03:00 is not supported without QTimeZone. Oppositely, if timezone feature is enabled (default), the example above produces expected result.