RFR: 8282567: Improve source-date handling in build system

[Magnus Ihse Bursie]

When doing reproducible builds, controlling the build time is imperative. To make this as easy as possible, some changes are needed in the build system.

 * If source-date is set to anything other than "updated" (meaning that it should be determined at build time), then the default value of --with-hotspot-build-time should be the same time.

* If the industry standard SOURCE_DATE_EPOCH is set when running configure, the default value for --with-source-date should be picked up from this variable. (If the command line option is given, it will override the environment variable however.)

* Finally the code can be made a bit clearer that we can set and export SOURCE_DATE_EPOCH to SOURCE_DATE in spec.gmk, unless we're using the "updated" (determined at build time) value.

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Commit messages:
 - Allow setting source date using SOURCE_DATE_EPOCH to configure
 - Fix else nesting
 - --source-date improvements

Changes: https://git.openjdk.java.net/jdk/pull/7660/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282567
  Stats: 67 lines in 5 files changed: 48 ins; 2 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7660.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7660/head:pull/7660

PR: https://git.openjdk.java.net/jdk/pull/7660