Check formatting in CI

.github/workflows/build.yml:
Use an Arch Linux container to check the formatting using clang-format.

.github/workflows/build.yml

@@ -9,6 +9,22 @@ on:
     branches: [main]
 
 jobs:
+  check_formatting:
+    runs-on: ubuntu-latest
+    container:
+      image: archlinux:latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install dependencies
+        run: pacman --noconfirm -Syu findutils git clang
+      - name: Run clang-format on all C and C++ sources
+        run: find . -type f -iname "*.c" -or -iname "*.cc" -or -iname "*.cpp" -or -iname "*.h" -exec clang-format -i {} \;
+      - name: Check whether there are differences
+        run: |
+          if [[ -n "$(git status -s)" ]]; then
+            printf "Files are not properly formatted!\n"
+            git diff
+          fi
   build_arch_linux_jack1:
     runs-on: ubuntu-latest
     container: