Make macOS build work on Apple Silicon (arm64) Homebrew

[jakeobsen]

Summary

The macOS bundling glue assumed Intel Homebrew's /usr/local prefix and hardcoded the wxWidgets 3.2 series, so on Apple Silicon (Homebrew at /opt/homebrew) the bundle silently produced a broken .app — no Frameworks/ libs were copied in, every wx translation was reported missing, and on a clean machine the resulting bundle crashed at launch with a missing @rpath/libsharpyuv.0.dylib.

This PR makes dist/osx/bundle.sh reproducible on both Intel and Apple Silicon Homebrew without hand-edits, and tolerates the wx 3.2 → 3.3 bump.

Changes

dist/osx/bundle.sh

• Discover BREW_PREFIX via brew --prefix and the wx series via wx-config --release, replacing the hardcoded /usr/local and 3.2 references.
• Export ACLOCAL_PATH so aclocal finds wxwin.m4 inside Homebrew's wxwidgets keg, removing the manual symlink workaround that the old README asked maintainers to do.
• Export CPPFLAGS/LDFLAGS for Homebrew's keg-only gettext, so configure doesn't disable NLS and silently drop the .gmo files (which then made the per-language copy step fail).
• Extend the dependency-walking loop to also resolve @rpath/... library references (e.g. libwebp → libsharpyuv) instead of only matching the Homebrew prefix; an unresolved @rpath ref previously slipped through and the resulting .app crashed at launch on a clean machine.

src/Makefile.am

• Pass $(AVFORMAT_CFLAGS)/$(AVCODEC_CFLAGS)/$(AVUTIL_CFLAGS) to the spek binary too, not just libspek.a. spek-spectrogram.cc transitively includes <libavutil/mem.h> via spek-fft.h; on Intel /usr/local/include was on clang's default search path so this was masked, but on Apple Silicon /opt/homebrew/include is not, and the build failed with 'libavutil/mem.h' file not found.

dist/osx/README.md

• Drop the manual /usr/local symlink and the hardcoded brotli install_name_tool stanza; bundle.sh now handles both automatically. Keep the brotli note as an example for the rare case where a Homebrew dylib still uses @loader_path siblings.

INSTALL.md

• Drop "Intel-based Mac" wording and reflect the current minimum versions (10.15+ on Intel per Info.plist, 11+ on Apple Silicon).

Backward compatibility

Fully backward-compatible on Intel: brew --prefix returns /usr/local there, wx-config --release returns whatever wx is installed, and the @rpath branch only fires for libs that actually use @rpath references. No behaviour change for an Intel maintainer running bundle.sh.

Dependency on #338

This PR alone is not sufficient to build against current Homebrew, because Homebrew now ships FFmpeg 8 which removed the avfft.h API src/spek-fft.cc uses. To verify these changes locally I applied #338 (hannesbraun:ffmpeg-8) on top of this branch — together they produce a working arm64 .app and .dmg. I deliberately did not include the FFmpeg 8 fix here to avoid duplicating an open PR.

Test plan

• ./dist/osx/bundle.sh runs to completion on macOS 26.4 / Apple Silicon (M5) with current Homebrew (FFmpeg 8.1, wxWidgets 3.3.2, gettext, automake) plus Fix compatibility with FFmpeg 8 #338 applied.
• lipo -archs dist/osx/Spek.app/Contents/MacOS/Spek reports arm64.
• otool -L on the binary and every bundled dylib shows no /opt/homebrew, /usr/local, or @rpath references — everything resolves to @executable_path/../Frameworks/....
• 26 wxstd-3.3.mo and 34 spek.mo translation files bundled.
• Ad-hoc-signed .app launches and stays running against tests/samples/2ch-44100Hz-16bps.wav.
• Spek.dmg (~19 MB) created.
• Cross-check on Intel Homebrew (no Intel Mac available to me; review of the diff should confirm brew --prefix → /usr/local is the only behavioural delta).