ci: Initial use of GitHub Actions.

This builds via cmake on Linux, macOS, and Windows. It doesn't yet
build the tools that use Qt or run tests.

.github/workflows/build.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on: [push, pull_request]
+
+env:
+  # Customize the CMake build type here (Release, Debug, RelWithDebInfo, etc.)
+  BUILD_TYPE: Release
+
+jobs:
+  build-cmake:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
+
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Create Build Environment
+      run: cmake -E make_directory ${{github.workspace}}/build
+
+    - name: Configure CMake
+      shell: bash
+      working-directory: ${{github.workspace}}/build
+      run: cmake $GITHUB_WORKSPACE -DCMAKE_BUILD_TYPE=$BUILD_TYPE
+
+    - name: Build
+      working-directory: ${{github.workspace}}/build
+      shell: bash
+      run: cmake --build . --config $BUILD_TYPE