Add CI with GitHub Actions

For now we only do a build and distcheck. The check target is also run
but there are no checks yet.

.github/workflows/build.yml

@@ -0,0 +1,29 @@
+name: CI
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        fetch-depth: 1
+    - name: Install FLTK
+      run: sudo apt install -y libfltk1.3-dev
+    - name: autogen
+      run: ./autogen.sh
+    - name: configure
+      run: ./configure
+    - name: make
+      run: make
+    - name: make check
+      run: make check
+    - name: make distcheck
+      run: make distcheck