add CI job to check coding style

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

.github/workflows/main.yml

@@ -34,3 +34,26 @@ jobs:
           # Ensure that "keg-only" Homebrew versions are used.
           PKG_CONFIG_PATH: "/usr/local/opt/ncurses/lib/pkgconfig:/usr/local/opt/expat/lib/pkgconfig:/usr/local/opt/curl/lib/pkgconfig:/usr/local/opt/openssl/lib/pkgconfig:/usr/local/opt/libffi/lib/pkgconfig:/usr/local/opt/sqlite/lib/pkgconfig:$PKG_CONFIG_PATH"
         run: ./ci-build.sh
+
+  code-style:
+    runs-on: ubuntu-20.04
+    name: Check coding style
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v2
+      - name: install dependencies
+        run: |
+          sudo apt update
+          sudo apt install -y --no-install-recommends autoconf autoconf-archive automake expect gcc git libcmocka-dev libcurl3-dev libgcrypt-dev libglib2.0-dev libgpgme11-dev libgtk2.0-dev libmicrohttpd-dev libncursesw5-dev libnotify-dev libotr5-dev libreadline-dev libsignal-protocol-c-dev libssl-dev libtool libxss-dev make pkg-config python3-dev python-dev-is-python3 libsqlite3-dev
+      - name: Install libstrophe
+        run: |
+          git clone https://github.com/strophe/libstrophe ../libstrophe
+          cd ../libstrophe && ./bootstrap.sh && ./configure && make -j$(nproc) && sudo make install
+      - name: Configure
+        run: |
+          ./bootstrap.sh
+          ./configure
+      - name: Check style
+        run: |
+          make format
+          git diff --exit-code