Ravenbrook developers can't update the Xcode project to alter the build for macOS

[rptb1]

None of the Ravenbrook developers have immediate access to macOS in order to maintain and update the Xcode project. For example, we are unable to extend it to include the extcon.c test in pull request #213 .

Various possible solutions:

1. Get a Mac for someone (could also solve M1 support issues), or rent a virtual Mac.
2. Use GNU Make to build on macOS, at least for CI purposes.
3. Use a cross-platform make-generator such as CMake (see #148 ).

[thejayps]

@rptb1 can this now be closed? I believe so.

[rptb1]

@rptb1 can this now be closed? I believe so.

Kinda. https://github.com/Ravenbrook/mps/pull/229 will resolve it properly.