On various 32 bit systems, time_t is actually 64 bits while long is
still only 32 bits. The macro would wrongly trigger its assertion in
this case if a value with more than 68 years worth of seconds is
converted.
Examples are various newer 32 bit platforms and old ones that are
compiled with -D_TIME_BITS=64.
Also statically assert that time_t is either 32 or 64 bits. Other values
might need adjustments in the macro.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6869>
