Add a GitHub action to avoid breaking builds

.github/actions/install_deps.sh

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+case `echo $1 | cut -d '-' -f 1` in
+        ubuntu)
+                sudo apt-get -qq update
+                sudo apt-get install -yq bison libpng-dev pkg-config libsdl2-dev
+                (
+                cd `mktemp -d`
+                curl -L https://github.com/rednex/rgbds/archive/v0.4.0.zip > rgbds.zip
+                unzip rgbds.zip
+                cd rgbds-*
+                make -sj
+                sudo make install
+                cd ..
+                rm -rf *
+                )
+                ;;
+        macos)
+                brew install rgbds sdl2
+                ;;
+        *)
+                echo "Unsupported OS"
+                exit 1
+                ;;
+esac

.github/workflows/buildability.yml

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+name: "Bulidability"
+on: push
+
+jobs:
+  buildability:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, ubuntu-16.04, macos-latest]
+        cc: [gcc, clang]
+        include:
+        - os: macos-latest
+          cc: clang
+          extra_target: cocoa
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install deps
+        shell: bash
+        run: |
+          ./.github/actions/install_deps.sh ${{ matrix.os }}
+      - name: Build
+        run: |
+          make sdl ${{ matrix.extra_target }} -j CONF=release CC=${{ matrix.cc }}
+      - name: Upload binaries
+        uses: actions/upload-artifact@v1
+        with:
+          name: sameboy-canary-${{ matrix.os }}-${{ matrix.cc }}
+          path: build/bin