RFR: 8287254: Clean up Xcode sysroot logic

[Erik Joelsson]

On Tue, 24 May 2022 17:09:10 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> The logic in BASIC_SETUP_DEVKIT for setting a correct sysroot for Xcode is hard to follow. This should be straightened out. We also expose variables that are no longer used. So there's a bit of related cleanup. 
> 
> The new code is more or less functionally equivalent to the old. There were some corner cases which the old code did not handle well; this has now been improved. I've also added yet another method of trying to get the SDK root before falling back to the system library, by using `xcrun --show-sdk-path`.
> 
> In an ideal world, the sysroot flags to `mig` should be set in configure, e.g. as `MIG_FLAGS` or `MIG_SYSROOT_FLAGS`. I can't be bothered to do that for a single call to `mig`, though.
> 
> As always, changes like this that depend on the environment is tricky to test. I've tried running it on a couple of different macs, with and without a devkit.

Marked as reviewed by erikj (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/8872