Target an even older version of Mac OS X

We can, so let's not ask if we should!

More realistically, we can right now so it's nice to mark that we do, but this is subject to be re-evaluated if any later changes conflicts with this.
Note also that this only affects the binaries we distribute with our releases!
It may very well be possible to compile RGBDS on even older versions of Mac OS X if you have the appropriate compiler setup;
we're just not testing it nor offering such binaries ourselves, but third-party packagers are welcome to do so.
(Looking at you, TigerBrew! ;)

.github/scripts/install_deps.sh

@@ -30,10 +30,18 @@ case "${OS%%-*}" in
 		sudo apt-get install -yq $pkgs
 		;;
 	macos)
+		pkgs=bison
+		case $TOOLSET in
+			'' | lld)
+				pkgs="$pkgs $TOOLSET"
+				TOOLSET=
+			;;
+		esac
 		# macOS bundles GNU Make 3.81, which doesn't support synced output.
 		# We leave it as the default in `PATH`, to test that our Makefile works with it.
 		# However, CMake automatically uses Homebrew's `gmake`, so our CI has synced output.
-		brew install bison make
+		# shellcheck disable=SC2086 # (This word splitting is intentional.)
+		brew install $pkgs make
 		# Export `bison` to allow using the version we install from Homebrew,
 		# instead of the outdated one preinstalled on macOS (which doesn't even support `-Wall`...).
 		export PATH="$(brew --prefix)/opt/bison/bin:$PATH"

.github/workflows/create-release-artifacts.yml

@@ -57,12 +57,14 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          ./.github/scripts/install_deps.sh macos
+          ./.github/scripts/install_deps.sh macos lld
       - name: Build binaries
         run: |
           cmake -S . -B build --preset macos-static -DFETCHCONTENT_BASE_DIR="${{ env.DEPS_ROOT_DIR }}"
           cmake --build build
           strip rgb{asm,link,fix,gfx}
+        env:
+          LDFLAGS: -fuse-ld=lld # cmake/macos-static.cmake comments explain why we use lld.
       - name: Package binaries
         run: |
           zip --junk-paths rgbds-macos.zip rgb{asm,link,fix,gfx} man/* .github/scripts/install.sh

.github/workflows/testing.yml

@@ -113,7 +113,7 @@ jobs:
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          .github/scripts/install_deps.sh macos
+          .github/scripts/install_deps.sh macos lld
       - name: Cache library deps
         uses: actions/cache@v5
         with:
@@ -124,6 +124,8 @@ jobs:
         run: |
           cmake -S . -B build --preset macos-static -DFETCHCONTENT_BASE_DIR="${{ env.DEPS_ROOT_DIR }}" -DTESTS_OS_NAME=macos
           cmake --build build
+        env:
+          LDFLAGS: -fuse-ld=lld # cmake/macos-static.cmake comments explain why we use lld.
       - name: Package binaries
         run: |
           mkdir bins

cmake/macos-static.cmake

@@ -2,10 +2,14 @@
 # in order to generate executables compatible with old macOS versions.
 # See our `macos-static` CMake preset for how it's meant to be used.
 
-# The `-mmacosx-version-min=10.9` flag ensures that the binary only uses APIs available on Mac OS X 10.9 Mavericks.
+# Note that targeting old enough versions of Mac OS X on recent enough versions of macOS
+# triggers some poorly-tested code paths within Apple's linker, which then crashes.
+# This can be worked around by using LLVM's LLD linker and passing `-fuse-ld=lld` when linking.
+
+# The `-mmacosx-version-min=10.4` flag ensures that the binary only uses APIs available on Mac OS X 10.4 Tiger.
 # The `-arch` flags build a "fat binary" that works on both Apple architectures:
 # older Intel x64 Macs and newer ARM "Apple Silicon" ones.
-set(secret_sauce -mmacosx-version-min=10.9 "SHELL:-arch x86_64" "SHELL:-arch arm64") # Avoid `-arch` being dedup'd.
+set(secret_sauce -mmacosx-version-min=10.4 "SHELL:-arch x86_64" "SHELL:-arch arm64") # Avoid `-arch` being dedup'd.
 add_compile_options(${secret_sauce})
 add_link_options(${secret_sauce})
 set(PNG_HARDWARE_OPTIMIZATIONS OFF) # These do not play well with a dual-arch build.