CI: test the build on Windows and macOS, too

Author: ctrueden

.github/workflows/build.yml

@@ -12,7 +12,11 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    name: build-${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
 
     steps:
       - uses: actions/checkout@v2