add GitHub Actions workflow file for Linux, macOS, and Windows

.github/workflows/build.yml

@@ -0,0 +1,46 @@
+name: build
+
+on:
+  push:
+  pull_request:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        include:
+          - name: Ubuntu 20.04
+            os: ubuntu-20.04
+            install_dir: ~/libebur128
+            cmake_extras: -DCMAKE_BUILD_TYPE=RelWithDebInfo
+          - name: macOS 10.15
+            os: macos-10.15
+            install_dir: ~/libebur128
+            cmake_extras: -DCMAKE_BUILD_TYPE=RelWithDebInfo
+          - name: Windows 2019
+            os: windows-2019
+            install_dir: C:\libebur128
+            cmake_extras: -DCMAKE_TOOLCHAIN_FILE=C:\vcpkg\scripts\buildsystems\vcpkg.cmake
+            cmake_config: --config RelWithDebInfo
+            ctest_config: --build-config RelWithDebInfo
+
+    name: ${{ matrix.name }}
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Check out Git repository
+      uses: actions/checkout@v2
+    - name: Configure
+      run: cmake -DCMAKE_INSTALL_PREFIX=${{ matrix.install_dir }} -DBUILD_SHARED_LIBS=ON
+           ${{ matrix.cmake_extras }} -S . -B build
+    - name: Build
+      run: cmake --build build ${{ matrix.cmake_config }}
+      env:
+        CMAKE_BUILD_PARALLEL_LEVEL: 2
+    # TODO: build and run tests and fuzzer
+    - name: Install
+      run: cmake --install build ${{ matrix.cmake_config }}
+    - name: Upload Build Artifact
+      uses: actions/upload-artifact@v2
+      with:
+        name: ${{ matrix.name }} libebur128 build
+        path: ${{ matrix.install_dir }}