Add GitHub Actions for continuous integration

.github/workflows/build.yml

@@ -0,0 +1,35 @@
+name: build
+on: [ push, pull_request ]
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [ clang++, gcc++, gcc-9++, gcc-10++ ]
+    steps:
+      - name: Add repository
+        run: sudo apt-add-repository -y "ppa:ubuntu-toolchain-r/test"
+      - name: Configure
+        run: cmake .
+      - name: Make
+        run: make
+        env:
+          CXX: {{ matrix.compiler }}
+      - name: Run tests
+        run: make check
+  macos:
+    runs-on: macos-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [ clang++, gcc++ ]
+    steps:
+      - name: Configure
+        run: cmake .
+      - name: Make
+        run: make
+        env:
+          CXX: {{ matrix.compiler }}
+      - name: Run tests
+        run: make check