RFR: 8276746: Add section on reproducible builds in building.md

[Erik Joelsson]

On Fri, 5 Nov 2021 16:30:26 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> Reproducible builds are all the vogue. The JDK has been making great strides in getting there, but still has some way to go. However, to get as close as possible, some special configuration is needed. 
> 
> This has been "tribal knowledge" of a few persons in the build team. It needs to be properly documented to help other users wanting to create reproducible builds of the JDK.

Marked as reviewed by erikj (Reviewer).

doc/building.md line 1551:

> 1549: 
> 1550: **Hint:** If your build environment already sets `SOURCE_DATE_EPOCH`, you can
> 1551: propagate this using `--with-source-date=$(SOURCE_DATE_EPOCH)`.

Shouldn't this example be $SOURCE_DATE_EPOCH, or at least ${SOURCE_DATE_EPOCH} as this would most likely be run in a shell? Or are you trying to send in the make variable reference verbatim to be evaluated in the makefiles?

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PR: https://git.openjdk.java.net/jdk/pull/6279