RFR: 8282567: Improve source-date handling in build system [v3]

[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.

Magnus Ihse Bursie has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into improve-source-date-in-configure
 - * Set copyright year from source-date.
   * Show source-date in summary.
   * Update build README on reproducible builds.
 - Allow setting source date using SOURCE_DATE_EPOCH to configure
 - Fix else nesting
 - --source-date improvements

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7660/files
  - new: https://git.openjdk.java.net/jdk/pull/7660/files/ee8899bf..864e17de

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=01-02

  Stats: 21169 lines in 498 files changed: 15222 ins; 3225 del; 2722 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