nixos (and probably some other distributives) place zoneinfo directory
not in /usr/share (in /etc for example). TZDIR is set accordingly.
Currently zoneinfo is looked for in /usr/share, disregarding TZDIR env
variable.

This commit adds compile definition for TZDIR if such env variable is
defined. This fixes zoneinfo lookup for nixos.

NO_CHANGELOG=build
NO_DOC=build
NO_TEST=build