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.

tools/check-sourcecode

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+# check-sourcecode -- check the avrdude source code for zombie mistakes
+# Copyright (C) 2024 Hans Ulrich Niedermann <hun@n-dimensional.de>
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+
+set -e
+
+prog="$(basename "$0")"
+cd "$(dirname "$0")"
+cd ..
+test -s README.md
+test -s COPYING
+test -s build.sh
+test -d .git/refs/heads
+
+
+declare -a checks=()
+fail=0
+succ=0
+
+
+checks+=(check_ac_cfg)
+check_ac_cfg() {
+    if git grep -E '#include\s+"ac_cfg\.h"'
+    then
+	echo "Error: Found #include \"ac_cfg.h\" with double quotes \"\". Should be <>."
+	echo "       See https://github.com/avrdudes/avrdude/issues/1706 for details."
+	return 1
+    fi
+}
+
+
+for check in "${checks[@]}"
+do
+    if "$check"; then
+	succ=$(( "$succ" + 1 ))
+	status="SUCC"
+    else
+	fail=$(( "$fail" + 1 ))
+	status="FAIL"
+    fi
+    echo "$status $check"
+done
+total=$(( "$succ" + "$fail" ))
+
+
+echo "$prog: Summary: $fail checks failed, $succ checks succeeded. $total checks in total."
+if [[ "$fail" -gt 0 ]]; then
+    exit 1
+else
+    exit 0
+fi