build(mac): Add -system-libs flag for Homebrew compatibility

Add a new `-system-libs` flag to mac/build.command that uses
system-installed libraries via pkg-config instead of bundled ones.
This enables Homebrew formula compatibility while preserving the
default standalone build behavior.

When `-system-libs` is passed:
- Uses pkg-config for: freetype2, gpac, libpng, libprotobuf-c,
  libutf8proc, zlib
- Does not compile bundled thirdparty sources
- Links against system libraries

Default behavior (no flag):
- Compiles bundled libraries as before
- No change to existing builds

Also adds a CI job `build_shell_system_libs` to test the new flag.

Refs #1580, #1534

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/build_mac.yml

@@ -42,6 +42,19 @@ jobs:
       with:
         name: CCExtractor mac build
         path: ./mac/artifacts
+  build_shell_system_libs:
+    # Test building with system libraries via pkg-config (for Homebrew formula compatibility)
+    runs-on: macos-latest
+    steps:
+    - name: Install dependencies
+      run: brew install pkg-config autoconf automake libtool tesseract leptonica gpac freetype libpng protobuf-c utf8proc zlib
+    - uses: actions/checkout@v6
+    - name: build with system libs
+      run: ./build.command -system-libs
+      working-directory: ./mac
+    - name: Display version information
+      run: ./ccextractor --version
+      working-directory: ./mac
   build_autoconf:
     runs-on: macos-latest
     steps: