git: Standardise non-root gitignore file format

Signed-off-by: Mark Grant <[email protected]>

m4/.gitignore

@@ -1,5 +1,20 @@
-# This .gitignore file is to ensure that this directory exists even if empty.
-# This is required because otherwise, after cloning, the project build will fail
-# citing this missing directory.
+#########################################################################
+#									#
+# Author: Copyright (C) 2023  Mark Grant				#
+#									#
+# Released under the GPLv3 only.					#
+# SPDX-License-Identifier: GPL-3.0-only					#
+#									#
+# N.B.									#
+# After changes to any gitignore file please run			#
+#		'git ls-files -i --exclude-standard'			#
+# to determine if any tracked files are now ignored.			#
+#									#
+#########################################################################
+
+# This .gitignore file is to ensure that this directory exists even though
+# none of it's contents will ever be tracked. This is required because
+# otherwise, after cloning, the project build will fail citing this missing
+# directory.
 # In other words, this is a git hack to satisfy AutoTools.