Jose-Luis Blanco-Claraco authored Oct 08, 2018 and Brad King committed Oct 09, 2018

The logic added by commit v3.6.0-rc1~338^2 (CMake: Extend TIMESTAMP
sub-commands with new unix time format specifier, 2016-02-16) to restore
the `TZ` environment variable does not properly handle the case in which
the variable was originally not set.  Unset the variable in this case.

Fixes: #18431