github actions: Add cmake build.

Copy cmake config from the ogg project for ci coverage on github.

.github/workflows/cmake.yml

@@ -0,0 +1,38 @@
+name: CMake build
+
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 0 1 * *'
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os:
+          [
+            ubuntu-latest,
+            macos-latest,
+            windows-latest,
+          ]
+
+    runs-on: ${{ matrix.os }}
+
+    env:
+      BUILD: _build
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Prepare build directory
+        run: mkdir ${{ env.BUILD }}
+
+      - name: Configure
+        run: cmake -S . -B ${{ env.BUILD }}
+
+      - name: Build
+        run: cmake --build ${{ env.BUILD }}
+
+      - name: Test
+        run: ctest --test-dir ${{ env.BUILD }} -V -C Debug