RFR: 8238650: Allow to override buildDate with SOURCE_DATE_EPOCH

[John Neffenger]

On Wed, 29 Jan 2020 08:58:47 GMT, Bernhard M. Wiedemann <github.com+637990+bmwiedemann at openjdk.org> wrote:

> Allow to override buildDate 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 PR was done while working on reproducible builds for openSUSE.

By the way, we have reproducible builds on Linux, macOS, and Windows now! (Well, still using `strip-nondeterminism` on the JMOD files, and as long as you build from the same file system path, thanks to `com.sun.javafx.css.parser.Css2Bin`. Sigh. This rabbit hole is deep.)

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PR: https://git.openjdk.java.net/jfx/pull/99