BUG: proper cross-compiling support for Arm

[nathanjhood]

Bug report

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• I confirm this is a bug with StoneyDSP, not with my own application or with JUCE.
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Describe the bug?

A clear and concise description of what the bug is.

Detailed steps on how to reproduce the bug?

The CI started failing just now for MacOS builds; a little deep deive into what has happened:

• I "fixed" the CMake Presets once I realised that I'd mistakenly used strings in the inheritance fields, instead of a string array
• once I amended the above, the "correct" behaviour kicked in, which populated all osx-based CMake Presets with CMAKE_OSX_ARCHITECTURES=x86_64;arm64 and VCPKG_OSX_ARCHITECTURES=x86_64;arm64 - this behaviour is just something I copied from JUCE and need to understand better...
• when the above two vars are correctly set, as they are now thanks to the above changes, the build step (looks like the linker, specifically) gets really confused about architectures, and fails when attempting to link an ARM-based installation of Catch2 to an x64-based plugin
• inspecting the linker args shows that both -arch x64 and -arch arm64 are being passed to the linker; no doubt, due to those vars above now being "correctly" defined...

What is the expected behaviour?

Link step should succeed and CI should pass.

Screenshots?

If applicable, add screenshots to help explain your problem.

Operating system(s), version(s), architectures?

MacOS x86_64 and ARM64

Additional context?

ideas

• easy solution - populate the *_OSX_ARCH vars with only a single arch per configure-preset
• medium solution - add another step to acquire the deps for both arch's on Mac, and hope the build is ok with it
• advanced solution - add hidden presets for x64 or arm64 as host arch, and x64 or arm64 as target arch, and build full cross-compiler support

[nathanjhood]

Here is the offending code from CMakePresets.json:

    // ...
      "cacheVariables": {
        "CMAKE_OSX_ARCHITECTURES": {
          "value": "x86_64;arm64",
          "type": "STRING"
        },
        "VCPKG_OSX_ARCHITECTURES": {
          "value": "x86_64;arm64",
          "type": "STRING"
        }
      },
      "environment": {
        "CMAKE_OSX_ARCHITECTURES": "x86_64;arm64",
        "VCPKG_OSX_ARCHITECTURES": "x86_64;arm64"
      }
    // ...

Some notes:

• SO article
• weird postfix with Python

All the above is for posterity. Testing removing the *_OSX_ARCHITECTURES vars temporarily...

[nathanjhood]

...and now, ARM-based Linux runner images for GitHub workflows.