[jdk20] RFR: 8298459: Fix msys2 linking and handling out of tree build directory for source zip creation [v3]

[Christoph Langer]

On Fri, 9 Dec 2022 14:58:16 GMT, Christoph Langer <clanger at openjdk.org> wrote:

>> This PR fixes two issues.
>> 
>> In ZipSource.gmk we don't account for the possibility of the build directory or `$(SUPPORT_OUTPUTDIR)` not being in the source directory tree (`$(TOPDIR)`).
>> It doesn't manifest in a build error though since link-file-relative will then fall back to absolute linking. But it results in awkward paths.
>> 
>> Furthermore, there's a lurking issue in msys2 builds on Windows. There, ln would create deep copies of a file system tree instead of linking. If the file system to be linked contains long paths, it could result in 'File name too long' errors, like can be seen [here](https://github.com/gdams/jdk11u-dev/actions/runs/3563481453/jobs/5986319107). We can avoid this by calling Windows mklink to create a file system junction. Presumably that would also help build performance (though not verified).
>
> Christoph Langer has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Play more with indentation

@erikj79 I would like to use these make conditionals but somehow it doesn't work out. What am I doing wrong, any idea/hint?

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PR: https://git.openjdk.org/jdk20/pull/9