Epic: Support reproducible builds

We currently distribute binary releases that people can use to download and install MacVim. We should add reproducible builds so that the binary we release is deterministic and reproducible and can be easily built on a local machine by any third party in an identical fashion.

Some motivations include the fact that supply chain attacks are real and while MacVim is unlikely a high-value target, it's still useful to reduce the likelihood of such problem happening to begin with. Also, with the deprecation of the macos-12 GitHub Action runner we will need to find another way to build the legacy MacVim binaries (either via a custom VM, or another CI service like Circle CI), and it would be nice to have some standard way to compare the generated artifacts and to have trust in the process working.

Note that we distribute signed apps, which is signed by a private key that only the maintainer controls, so it's impossible to have an artifact (MacVim.app) that can be compared bit-by-bit to a local build. We will need to provide a script that could strip the app signature before comparing.

• Use relative paths for debug symbols and reproducible linker flags when building Vim
• Use SOURCE_DATE_EPOCH to make builds not dependent on date/time.
• Add documentation, and refactor build scripts (also necessary when adding another CI system) to make it more easily to build locally with the same flags.
• Add CI output artifacts that contain necessary environment info.
• #1586
• Add user script that could compare two apps by stripping the signature, resign with adhoc signature, and then compare the result binaries. We also need to compare the entitlements.
• CI test to validate reproducible builds.
• Test in older versions of Xcode to make sure didn't break anything.

Related:

• Add support for exporting dSYM files as part of build steps. These contain debug symbols and help diagnose crash issues for users. Make sure to use relative paths as it's necessary for reproducibility. This is a bit annoying to do due to a clang issue llvm/llvm-project#113973.

El issue no especifica archivos ni pruebas concretos; empieza revisando los scripts de build existentes y el binary-release CI workflow. Se considera terminado cuando las compilaciones locales y de CI utilizan flags y datos de entorno coherentes, se pueden comparar apps sin firmar o firmadas ad-hoc, y CI valida la reproducibilidad en los entornos de build compatibles.