RFR: 8275745: Reproducible copyright headers

[Emmanuel Bourg]

The copyright headers are generated at build time, and the year inserted in the template depends on the current date. This means the headers are not reproducible if the project is built a year later. The year in the headers could be derived from the SOURCE_DATE_EPOCH environment variable to make the build reproducible (this variable is already used in other parts of the build).

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Commit messages:
 - Make the copyright headers reproducible when the SOURCE_DATE_EPOCH environment variable is defined

Changes: https://git.openjdk.java.net/jdk/pull/1498/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1498&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275745
  Stats: 24 lines in 2 files changed: 18 ins; 2 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1498.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1498/head:pull/1498

PR: https://git.openjdk.java.net/jdk/pull/1498