RFR: 8288001: SOURCE_DATE_EPOCH should not be set by default

[KIRIYAMA Takuya]

At default configuration, SOURCE_DATE_EPOCH is exported as environment variable in SetupReproducibleBuild. Then, gcc is affected of SOURCE_DATE_EPOCH environment variable. This value is used only to set SOURCE_DATE_ISO_8601 (except below), so I removed "export" for SOURCE_DATE_EPOCH in SetupReproducibleBuild. And, at building ct.sym, SOURCE_DATE_EPOCH environment variable is needed. So I added setting routine if SOURCE_DATE_EPOCH is not set.

This fix works fine. With default configuration shows -Xinternalversion output same as Windows, and with --with-source-date configuration shows -Xinternalversion output specified timestamp. Would you please review this fix?

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Commit messages:
 - 8288001: SOURCE_DATE_EPOCH should not be set by default
 - 8288001: SOURCE_DATE_EPOCH should not be set by default

Changes: https://git.openjdk.java.net/jdk/pull/9081/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288001
  Stats: 5 lines in 2 files changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9081.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9081/head:pull/9081

PR: https://git.openjdk.java.net/jdk/pull/9081