RFR: 8238650: Allow to override buildDate with SOURCE_DATE_EPOCH

[Bernhard M.Wiedemann]

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.

-------------

Commits:
 - e7f71fb9: 8238650: Allow to override buildDate with SOURCE_DATE_EPOCH

Changes: https://git.openjdk.java.net/jfx/pull/99/files
 Webrev: https://webrevs.openjdk.java.net/jfx/99/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8238650
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/99.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/99/head:pull/99

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