MacOS build

[dredozubov]

Add macOS Build Support & Modernize CMake

Summary

• Add macOS CI build job for VST3 and Audio Unit plugins (Universal Binary)
• Modernize CMake to use FetchContent for automatic dependency management
• Add comprehensive build documentation

Changes

CMakeLists.txt

• Replace manual ../JUCE and ../AlgLib paths with CMake FetchContent
• Dependencies are now downloaded automatically during configure
• No external setup required - just clone and build

CI Pipeline (.github/workflows/build.yml)

• Add build-macos job running on macos-latest
• Build Universal Binary (x86_64 + arm64) for Intel and Apple Silicon
• Upload VST3 and AU plugin artifacts

Documentation

• Add BUILDING.md with prerequisites and build instructions for macOS, Linux, and Windows
• Include installation paths for each platform

.gitignore

• Add build/ directory

Test Plan

• Verify macOS CI build completes successfully
• Verify Linux CI build still works with FetchContent
• Verify Windows CI build still works

[justonem0reuser]

Thank you for the contribution - this is a really great improvement!

I'll merge it and then apply a follow-up commit to:

• fetch external dependencies into external folders to make them reusable;
• use raw download and unzip commands for AlgLib to prevent CMake files creating and keep both JUCE and AlgLib in theirs version-specific folders;
• add running an external script (if exists) to the "install" target - for example, to run a DAW;
• avoid caching JUCE build on Linux to prevent external dependency versions conflicts (cached JUCE vs MatchingCompressor);
• add VST3 installer for Windows artefact using Inno Setup;
• add a job for MasOS dev-only release.

I'll document these changes in more detail in the follow-up commit.

Thanks again!