Enable -Wdate-time warning

This produces an "expansion of date or time macro is not reproducible"
warning or error upon use of DATE or TIME.

Provide NO_WDATE_TIME for ports or 3rd party software to opt out.

This is a recommit of 489d7a8528ca, which was reverted (by baf8cbcd97a6)
due to ports build failures.

PR: 267902 [exp-run]
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29580