openjdk25: print actionable error when Metal toolchain is not set up correctly for use with MacPorts

[breun]

Description

Print an actionable error message when the Metal toolchain is not set up correctly for use with MacPorts.

Type(s)

• enhancement

Tested on

macOS 26.5 25F71 arm64
Xcode 26.5 17F42

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[breun]

@markmentovai Does this PR look ok to you? GitHub Actions builds for macOS 26 fail, because the Metal toolchain is not present in this environment, but at least that demonstrates that new more actionable error message. (The MacPorts buildbots do have the Metal toolchain installed.)

[markmentovai]

Off to a good start.

[ryandesign]

GitHub Actions builds for macOS 26 fail, because the Metal toolchain is not present in this environment,

Really? I thought GitHub Actions had the Metal toolchain installed since a few months ago.

[Dave-Allured]

Really? I thought GitHub Actions had the Metal toolchain installed since a few months ago.

Yes this was supposed to be solved for GitHub Actions. However, recall that GitHub was quite flaky about Metal, earlier in this saga.

In PR 32402 I offered a simple and obvious diagnostic for the Metal installation at GitHub (CI) bootstrap time. It may be time to reconsider this for temporary or permanent use. You could insert this temporarily into your your own workflows, if that would be of any help. Bootstrap.sh is only for GitHub runners, so this does not show anything for MacPorts builders.

[breun]

GitHub Actions builds for macOS 26 fail, because the Metal toolchain is not present in this environment,

Really? I thought GitHub Actions had the Metal toolchain installed since a few months ago.

That comment was from a couple of months ago. GitHub Actions builds for macOS 26 now succeed indeed.

[breun]

@ryandesign @Dave-Allured I'd like to merge this, and then as a next step I'd like to extract the pre-configure checks so they can be reused by the other openjdk* ports and any other ports that require the Metal toolchain to build. Does it make sense to you to introduce a metal PortGroup for this?

[Dave-Allured]

@breun Outdated comment: Oops. You can ignore my last suggestion.

PortGroup: No strong opinion. I am not the right person to ask. Use your own judgement. If it makes things generally easier for users and port maintainers, then yes, go ahead.