Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

Allow to override build date with SOURCE_DATE_EPOCH in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

This patch allows openSUSE's resteasy package to build more reproducibly.

Without this patch, there were such diffs:

+++ /usr/share/javadoc/resteasy/org/jboss/resteasy/client/jaxrs/i18n/LogMessages_$logger.html       2020-05-09 00:52:40.125364506 +0000
@@ -143,7 +143,7 @@
 </dl>
 <hr>
 <pre><a href="https://docs.oracle.com/javase/7/docs/api/javax/annotation/Generated.html?is-external=true" title="class or interface in javax.annotation" class="externalLink">@Generated</a>(<a href="https://docs.oracle.com/javase/7/docs/api/javax/annotation/Generated.html?is-external=true#href_anchor" title="class or interface in javax.annotation" class="externalLink">value</a>="org.jboss.logging.processor.generator.model.MessageLoggerImplementor",
-           <a href="https://docs.oracle.com/javase/7/docs/api/javax/annotation/Generated.html?is-external=true#href_anchor" title="class or interface in javax.annotation" class="externalLink">date</a>="2020-05-09T00:51:48+0000")
+           <a href="https://docs.oracle.com/javase/7/docs/api/javax/annotation/Generated.html?is-external=true#href_anchor" title="class or interface in javax.annotation" class="externalLink">date</a>="2020-05-09T00:53:02+0000")

This PR was done while working on reproducible builds for openSUSE.

[jamezp]

Sorry this is over a year late, but I just noticed this PR. Is there a reason for this? Seems odd to me.

[bmwiedemann]

Hi. https://reproducible-builds.org/docs/buy-in/ lists several good reasons for wanting deterministic build results.
Ideally we want to be able to get identical results anytime anywhere (given the right build-env)

[jamezp]

While I understand the reasoning I don't think we should allow for the date to be overridden. We could however leave the date off, e.g. return an empty string. Manipulating the actual date seems wrong to me though.

[bmwiedemann]

Sure, if there is nothing that expects the date to exist, omitting it can also work.