[PATCH v2] m4: make serial numbers effective

Similar to a reported issue in gnulib: https://lists.gnu.org/archive/html/bug-gnulib/2024-04/msg00051.html.

Adhrere to the M4 serial file format documented at https://www.gnu.org/software/automake/manual/html_node/Serials.html.

* m4/bison-check-compiler-flag.m4: Make formatting consistent.
* m4/bison-cxx-std.m4: Make serial number effective.
* m4/bison-i18n.m4: Ditto.
* m4/flex.m4: Ditto.

Copyright-paperwork-exempt: yes
---
v2: Crank serial number for changes too. This makes life a lot easier if
    trying to detect changes because you can assume that sensible macros
    map serial:checksum 1:1. Thanks to Eric Gallager for reporting.

 m4/bison-check-compiler-flag.m4 | 3 +--
 m4/bison-cxx-std.m4             | 3 ++-
 m4/bison-i18n.m4                | 3 ++-
 m4/flex.m4                      | 3 ++-
 4 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/m4/bison-check-compiler-flag.m4 b/m4/bison-check-compiler-flag.m4
index 89853069..d636a8bf 100644
--- a/m4/bison-check-compiler-flag.m4
+++ b/m4/bison-check-compiler-flag.m4
@@ -1,6 +1,5 @@
 # bison-check-compile-flag.m4
-
-# serial 1
+# serial 2
 
 # Copyright (C) 2018-2022 Free Software Foundation, Inc.
 
diff --git a/m4/bison-cxx-std.m4 b/m4/bison-cxx-std.m4
index cc7fea19..6c6c4a6d 100644
--- a/m4/bison-cxx-std.m4
+++ b/m4/bison-cxx-std.m4
@@ -1,4 +1,5 @@
-# bison-cxx-std.m4 serial 1
+# bison-cxx-std.m4
+# serial 2
 
 # Copyright (C) 2018-2022 Free Software Foundation, Inc.
 
diff --git a/m4/bison-i18n.m4 b/m4/bison-i18n.m4
index 24d53991..1d15a9dc 100644
--- a/m4/bison-i18n.m4
+++ b/m4/bison-i18n.m4
@@ -1,4 +1,5 @@
-# bison-i18n.m4 serial 2
+# bison-i18n.m4
+# serial 3
 
 dnl Copyright (C) 2005-2006, 2009-2015, 2018-2022 Free Software
 dnl Foundation, Inc.
diff --git a/m4/flex.m4 b/m4/flex.m4
index c5ed43ac..23b641cb 100644
--- a/m4/flex.m4
+++ b/m4/flex.m4
@@ -1,4 +1,5 @@
-# flex.m4 serial 2
+# flex.m4
+# serial 3
 # Copyright (C) 2012-2015, 2018-2022 Free Software Foundation, Inc.
 # This file is free software; the Free Software Foundation
 # gives unlimited permission to copy and/or distribute it,
-- 
2.44.0