RFR: 8282567: Improve source-date handling in build system [v2]

[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 incrementally with one additional commit since the last revision:

  * Set copyright year from source-date.
  * Show source-date in summary.
  * Update build README on reproducible builds.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7660/files
  - new: https://git.openjdk.java.net/jdk/pull/7660/files/f8d5dbb7..ee8899bf

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=00-01

  Stats: 118 lines in 3 files changed: 66 ins; 30 del; 22 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