Use SOURCE_DATE_EPOCH instead of current year in copyright message to…

[fasterit]

… make build reproducible

Debian patch from
https://anonscm.debian.org/cgit/collab-maint/htop.git/tree/debian/patches/003-use-source-date-epoch.patch
by Graham Inggs

[fasterit]

Alexis Bienvenüe has proposed a similar patch in Debian bug #822963 that may be more robust on BSDs. Could be worth checking on these platforms. Feedback welcome here and/or on the Debian bug report.
The essence is:

+-year=$(date +%Y)
++SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(date +%s)}"
++year=$(date -u -d "@$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u -r "$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u "+%Y")

... to make htop 2.0.1+ build reproducibly.
Also see https://tests.reproducible-builds.org/rb-pkg/unstable/amd64/htop.html while builds are still unreproducible.

[hishamhm]

@fasterit Thank you! I'll use these two lines from your last comment.