[gimp] icons, tools: add a comment to generated icon-list.mk.

[Jehan <jehanp src gnome org>]

commit 7a2f4b82f06545eafe08c3e87a6773ae936e8a50
Author: Jehan <jehan girinstud io>
Date:   Thu Jan 27 00:59:05 2022 +0100

    icons, tools: add a comment to generated icon-list.mk.
    
    Just so that someone who happens to look in there is not tempted to
    modify these files directly and get some instructions.
    The comment is inspired from other similarly generated Makefile.am (i.e.
    in plug-ins/commons/).

 icons/Color/icon-list.mk         |  8 ++++++++
 icons/Symbolic/icon-list.mk      |  8 ++++++++
 tools/generate-icon-makefiles.py | 11 +++++++++++
 3 files changed, 27 insertions(+)
---
diff --git a/icons/Color/icon-list.mk b/icons/Color/icon-list.mk
index c2c54b7987..94cb8659d1 100644
--- a/icons/Color/icon-list.mk
+++ b/icons/Color/icon-list.mk
@@ -1,3 +1,11 @@
+
+## --------------------------------------------------------------
+## This file is autogenerated by tools/generate-icon-makefiles.py
+## --------------------------------------------------------------
+
+## Modify this script or files inside icons/icon-lists/ instead of this one.
+## Then run tools/generate-icon-makefiles.py again.
+
 scalable_images = \
        scalable/dialog-error.svg                               \
        scalable/dialog-information.svg                         \
diff --git a/icons/Symbolic/icon-list.mk b/icons/Symbolic/icon-list.mk
index 356a48c73c..24326a7a9a 100644
--- a/icons/Symbolic/icon-list.mk
+++ b/icons/Symbolic/icon-list.mk
@@ -1,3 +1,11 @@
+
+## --------------------------------------------------------------
+## This file is autogenerated by tools/generate-icon-makefiles.py
+## --------------------------------------------------------------
+
+## Modify this script or files inside icons/icon-lists/ instead of this one.
+## Then run tools/generate-icon-makefiles.py again.
+
 scalable_images = \
        scalable/dialog-error-symbolic.svg                                      \
        scalable/dialog-information-symbolic.svg                                \
diff --git a/tools/generate-icon-makefiles.py b/tools/generate-icon-makefiles.py
index 24d6f77d20..7f5be9f493 100755
--- a/tools/generate-icon-makefiles.py
+++ b/tools/generate-icon-makefiles.py
@@ -64,6 +64,17 @@ def print_icons(indir, filenames, max_len, prefix, suffix, outfile, endlist=True
 if __name__ == "__main__":
 
   with open(color_mk, mode='w') as colorf, open(symbolic_mk, mode='w') as symbolicf:
+    top_comment = '''
+## --------------------------------------------------------------
+## This file is autogenerated by tools/generate-icon-makefiles.py
+## --------------------------------------------------------------
+
+## Modify this script or files inside icons/icon-lists/ instead of this one.
+## Then run tools/generate-icon-makefiles.py again.
+'''
+    print(top_comment, file=colorf)
+    print(top_comment, file=symbolicf)
+
     # Scalable icons.
     print("scalable_images = \\", file=colorf)
     print("scalable_images = \\", file=symbolicf)