RFR: 8238650: Allow to override buildDate with SOURCE_DATE_EPOCH

[Bernhard M.Wiedemann]

On Wed, 29 Jan 2020 12:33:00 GMT, Kevin Rushforth <kcr 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.
> 
> In addition to a signed OCA, we will need a bug filed to track this. Please read the [CONTRIBUTING guidelines](https://github.com/openjdk/jfx/blob/master/CONTRIBUTING.md).

/signed

I filed one bug and got review ID : 9063488

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