Allow builds to be reproducible by default

[bmwiedemann]

This is a follow-up to PR #2 :

Allow builds to be reproducible by default
without specifying project-specific configure flags
to give distribution packagers an easier time.

If you do not set SOURCE_DATE_EPOCH, it will still include the build date as before.

Note, that this change is split into 2 commits.

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

[hb9xar]

I quickly checked the specification at https://reproducible-builds.org/specs/source-date-epoch/
To my understanding, if SOURCE_DATE_EPOCH is passed, the build process must use this value for build timestamps, so using "unknown" would not really comply.
If SOURCE_DATE_EPOCH is set, the build process should:

• not use BUILDSTR (resp. set it to "none")
• use the supplied timestamp to create a BUILDDATE (in UTC timezone)

[bmwiedemann]

It is OK to keep things simple and just use it as a flag here. The important thing is that it is possible to get identical binaries twice.

[hb9xar]

Implemented with 4750bea
BUILDDATE taken from env var SOURCE_DATE_EPOCH if present.

[bmwiedemann]

Looks good now