ci: Add macOS build to GitHub Actions

The codebase already has macOS support (dylib flags in the Makefile,
macOS linker handling in meson.build), but CI only tests Linux,
Windows, and FreeBSD.  This means macOS regressions go unnoticed
until a user reports them.

Add a build-macos job that tests both Make and Meson builds on
macos-latest.  The runner ships with clang, flex, bison, python3,
and git; only libyaml, swig, meson, ninja, and pkg-config are
installed via Homebrew.

Apple Clang uses -Wsign-compare by default, which trips on some lex
generated code.  Suppress the warning for the two flex-generated
objects.

We skip tests for now, because trees.S doesn't work with the MacOS
assembler.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

.github/workflows/build.yml

@@ -71,6 +71,37 @@ jobs:
       - name: Run check
         run: if ! meson test -C build; then cat build/meson-logs/testlog.txt; false; fi
 
+  build-macos:
+    runs-on: macos-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        build: [ "make", "meson" ]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Install Dependencies
+        run: |
+          brew install libyaml swig meson ninja pkg-config
+
+      - name: Build (Make)
+        if: matrix.build == 'make'
+        run: |
+          make
+
+      - name: Setup (Meson)
+        if: matrix.build == 'meson'
+        run: meson setup -D python=enabled -D yaml=enabled -D tests=false build
+
+      - name: Build (Meson)
+        if: matrix.build == 'meson'
+        run: meson compile -C build
+
   build-windows:
     runs-on: windows-latest
     strategy:

Makefile

@@ -153,6 +153,11 @@ include Makefile.convert-dtsv0
 include Makefile.dtc
 include Makefile.utils
 
+# Flex-generated source triggers -Wsign-compare in its templates
+dtc-lexer.lex.o: CFLAGS += -Wno-sign-compare
+convert-dtsv0-lexer.lex.o: CFLAGS += -Wno-sign-compare
+
+
 BIN += convert-dtsv0
 BIN += dtc
 BIN += fdtdump