RFR: 8282567: Improve source-date handling in build system [v6]

[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:

  Fix building.md

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7660/files
  - new: https://git.openjdk.java.net/jdk/pull/7660/files/b97625fd..fd2ba3c6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7660&range=04-05

  Stats: 7 lines in 2 files changed: 0 ins; 0 del; 7 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