Attempt to do cross-android build in CI

.github/workflows/ubuntu.yml

@@ -19,3 +19,15 @@ jobs:
       run: sudo apt install check
     - name: run tests
       run: make test
+
+  build-android:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - uses: nttld/setup-ndk@v1
+      with:
+        ndk-version: r21e
+    - name: make
+      run: make cross-android