The tranditional build makes multiple passes through the tree.
The DIRDEPS_BUILD visits each directory only once per architecture,
thus makefiles should be able to everything they need in a single pass.
The use of TZS!= when doing make(*install*)
only works if the directory has previously been visited to do zoneinfo
since before the zoneinfo target is run TZS will be empty.
To fix this, have the zoneinfo target capture the list of files to
zoneinfo, and install-zoneinfo use that list.
This is more efficient - we only gather the list of zones when it is
likely to have changed, and allows the makefile to do everything in a
single pass.