RFR: JDK-8282532: Allow explicitly setting build platform alongside --openjdk-target [v4]

[Magnus Ihse Bursie]

On Thu, 3 Mar 2022 08:12:37 GMT, Julian Waters <jwaters at openjdk.org> wrote:

>> Currently the only other option for manually configuring the build platform while cross compiling are devkits, which don't work on certain systems and are also more focused on differentiating the build and target compilers instead. This patch adds the ability to explicitly set the build platform through a new option, which can be especially helpful for when autodetection fails and devkits cannot be relied on, and also for simpler cross compilation cases (Like the one described in building.md)
>> 
>> WIP: Translation from markdown to html
>
> Julian Waters has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Syntax

This is looking much better. Apart from the "unsafe" comment, the only thing remaining is that you update building.html by running `make update-build-docs`.

make/autoconf/configure line 298:

> 296:     exit 1
> 297:   else
> 298:     echo "Warning: You are using unsafe autoconf cross-compilation flags."

The autoconf flags are not really unsafe, they are just misleadingly named. I'd rather see that you restore "legacy" in this output, and the conf_legacy_crosscompile variable name.

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

Changes requested by ihse (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7656