Add initial CI via GitHub Actions. (#22)

This does a basic build for macOS Catalina and Ubuntu Bionic.

.github/workflows/build.yml

@@ -0,0 +1,22 @@
+name: Build
+
+on: [push, pull_request]
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-10.15, ubuntu-18.04]
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install X11 dependencies on MacOS
+        if: ${{ runner.os == 'macOS'}}
+        run: brew cask install xquartz
+      - name: Build
+        working-directory: bin
+        run: ./makeright x