Get Linux compilation working on GitHub Actions

.github/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: ci
+
+on:
+  push:
+    branches:
+      - macos-ci
+
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  compile-linux:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Install Ubuntu packages
+      run: grep 'sudo apt install' INSTALL | sed 's/\$//' | sh
+    - name: Compile
+      run: make -C GUI/xephem