Timezone: resolve $TZDIR in runtime

Hello,

I was recently working with a project which embeds postgres into
application. It uses binaries built for Debian, and configured to look for
timezones in Debian's timezone folder /usr/share/zoneinfo/, which wasn't
present on my system.

I was thinking if it's possible to redefine it in runtime, and found that
it is not possible: postgres support either compilation flag
--with-system-tzdata or built-in tz file -- timezone/data/tzdata.zi. From
the other hand, looking at src/timezone/pgtz.c#L54, it seems that timezone
dir resolve happens in runtime, which means, it is possible to use $TZDIR
environment variable or configuration option to achieve this without
recompilation.

Do you think it is a feasible thing to do? it would be very convenient for
portable postgers builds.

Thanks!