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

[Erik Joelsson]

On Wed, 2 Mar 2022 16:52:12 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.

I think building.md needs to be updated to reflect these changes, as you no longer need to manually propagate the value of SOURCE_DATE_EPOCH through --with-source-date as the documentation currently suggests.

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