Allow to override build date with SOURCE_DATE_EPOCH

[bmwiedemann]

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated master/src/CHANGES.txt directory (and read the README.txt in that directory)
• I have updated the appropriate documentation

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.

Also consistently use ISO 8601 date format
to be understood everywhere.
Also use gmtime to be independent of timezone.

[bdbaddog]

Please update CHANGES.txt

So if SOURCE_DATE_EPOCH is not set, this is still changing the default format of the time/datestamp in the distibuted code right?

[bdbaddog]

Also, am I understanding the motivation correctly.
That since binary distributions are produced that setting SOURCE_DATE_EPOCH you can eliminate any datestamps from being changes and so a simple (binary?) diff of packages could ensure that the package hasn't been altered

[bmwiedemann]

The results should be bit-identical, so you can just compare the hash/digest of the resulting packages. We are nearly there (e.g. it is working for 95% of 11000 openSUSE packages already)

Yes, it would change the date format. If there are many parsers around, we could leave the SConstruct version as is, but I guess, few people care about the man-page header.

[bdbaddog]

o.k. I'm going to leave this up for a few days and float the PR to our users mailing list.
I can't think of any good reason not to merge this though, but I like to give our users a heads up.
Maybe add in the CHANGES.txt that the effect will be datestamps in docs and embedded in code will change format even when SOURCE_DATE_EPOCH is not specified?

[bmwiedemann]

The man-page format is also particularly hard to understand (without looking at the source) - e.g. is 01/02/2018 January or February?

Will add a CHANGES.txt entry.

[bdbaddog]

US date formatting so January.

[bmwiedemann]

In Germany, 01.02.2018 is the first of February and I don't want to know how many other variants exist around the world.

[bdbaddog]

Indeed. One of many locale based date/time/etc variations.

[coveralls]

Coverage remained the same at 79.953% when pulling a61f52d on bmwiedemann:date into 0c20ae2 on SCons:master.

[bdbaddog]

Please add a note that this is only about building SCons packages and not universal to builds done by scons when building other software.