... MK_ZONEINFO_LEAPSECONDS_SUPPORT == yes and
MK_ZONEINFO_OLD_TIMEZONES_SUPPORT == yes.
Keep LEAPSECONDS and OLDTIMEZONES for backwards compatibility, but print out a
warning notifying users that they should use the new variables, in an effort to
migrate them to the variables. This is being done mostly for automated build
tools, etc, that might rely on these variables being set. The variables will be
removed in the future on ^/head, e.g., after ^/stable/12 is cut.
MFC after: 1 month
Relnotes: yes