Add automated Linux build script

Not providing downloads at this point, but at least I'll know when I break the build...

.github/workflows/linux-build.yml

@@ -0,0 +1,38 @@
+name: Linux Build
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        submodules: true
+
+    - name: Install packages
+      shell: bash
+      run: sudo apt-get -y install libsdl2-dev libgtk2.0-dev
+
+    - name: Compile debug build
+      shell: bash
+      run: |
+        mkdir build-debug
+        cd build-debug
+        cmake -DCMAKE_BUILD_TYPE=Debug ..
+        make
+        
+    - name: Compile release build
+      shell: bash
+      run: |
+        mkdir build-release
+        cd build-release
+        cmake -DCMAKE_BUILD_TYPE=Release ..
+        make
+