Make the build reproducible (#6817)

Whilst working on the Reproducible Builds effort [0] we noticed that
Chart.js could not be built reproducibly.

This is because it was embedding another build date in a "banner"
comment. This patch uses the value from SOURCE_DATE_EPOCH [1] if
present.

This was originally filed in Debian as #946333 [2].

 [0] https://reproducible-builds.org/
 [1] https://reproducible-builds.org/specs/source-date-epoch/
 [2] https://bugs.debian.org/946333

Signed-off-by: Chris Lamb <lamby@debian.org>