ExternalProject: Fix misuse of IS_NEWER_THAN in timestamp checks
This change was originally made in 404cddb7bb (ExternalProject:
Fix misuse of IS_NEWER_THAN in timestamp checks, 2021-02-21),
but was reverted by 57d442e182 (Revert ExternalProject and
FetchContent refactoring, 2021-03-10) due to regressions from
other changes. Reapply this fix on its own, since it is still valid.
When using a file system which only has second resolution timestamps,
there is a reasonably high likelihood of timestamps being the same.
The IS_NEWER_THAN test returns true when timestamps are the same,
so don't redo downloads when they match exactly.
1 file changed
