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

[Erik Joelsson]

On Mon, 7 Mar 2022 17:23:40 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> 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

Marked as reviewed by erikj (Reviewer).

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