Add checks for zombie sourcecode problems

This adds checks for zombie sourcecode problems, i.e. problems
we have tried to eliminate but which might be or have been
resurrected.

The example check is for the problematic

    #include "ac_cfg.h"

with double quotes instead of <> which were identified as a
problem in https://github.com/avrdudes/avrdude/issues/1706
and then fixed.

.github/workflows/check-sourcecode.yml

@@ -0,0 +1,24 @@
+name: "Check Sourcecode"
+
+on:
+  push:
+    branches-ignore:
+      - 'onlinedocs'
+  pull_request:
+    branches-ignore:
+      - 'onlinedocs'
+
+jobs:
+
+  check-sourcecode:
+    name: "Check Sourcecode"
+    runs-on: ubuntu-latest
+
+    steps:
+
+      - uses: actions/checkout@v4
+
+      # - name: Install prerequisites
+
+      - name: "Check the sourcecode"
+        run: ./tools/check-sourcecode