CI macOS: build with ECCE_USE_COIN against Homebrew's coin3d (#166, #133)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R9WkbXZ96rqCJxYuumcrgF

Author: andy

.github/workflows/build.yml

@@ -245,9 +245,12 @@ jobs:
           # equivalent and doesn't ship X11 at all by default. XQuartz
           # is the standard way to get real X11/Xt headers+libs on macOS.
           brew install --cask xquartz
+          # The vendored Inventor core needs GLX and pre-C++17 code that
+          # Apple's compiler rejects; the Coin build (#166) skips it.
+          brew install coin3d || { echo "::error::Homebrew has no coin3d formula"; exit 1; }
 
       - name: Configure
-        run: cmake -G Ninja -B build-cmake -DCMAKE_PREFIX_PATH=/opt/X11
+        run: cmake -G Ninja -B build-cmake -DECCE_USE_COIN=ON -DCMAKE_PREFIX_PATH="/opt/X11;$(brew --prefix)"
 
       - name: Build
         run: cmake --build build-cmake