[tz] make rearguard_tarballs fails on macOS

[Deborah Goldsmith]

Hi,

The change to Makefile for 2020c makes it fail on macOS, as the touch command on Darwin does not have a -d option:

> : The dummy pacificnew pacifies TZUpdater 2.3.1 and earlier.
> touch -md 2020-10-12T22:53:00Z \
> 		  tzdata2020c-rearguard.dir/pacificnew
> touch: illegal option -- d
> usage:
> touch [-A [-][[hh]mm]SS] [-acfhm] [-r file] [-t [[CC]YY]MMDDhhmm[.SS]] file ...
> make[1]: *** [tzdata2020c-rearguard.tar.gz] Error 1
> make: *** [rearguard_tarballs] Error 2

Debbie