Process C23 #elifdef and #elifndef

Inspired by Collin Funk’s suggestion in:
https://lists.gnu.org/r/autoconf-patches/2024-06/msg00000.html
* bin/autom4te.in (handle_output):
* bin/autoscan.in, bin/ifnames.in (scan_c_file):
Also recognize #elifdef and #elifndef.
* tests/tools.at (ifnames): Test it for ifnames.

NEWS

@@ -22,6 +22,12 @@ GNU Autoconf NEWS - User visible changes.
  This reverts to Autoconf 2.69 behavior, and also fixes a bug where
  AC_PROG_CXX rejected C++20 compilers.
 
+** New features
+
+*** Programs now recognize #elifdef and #elifndef.
+ The autom4te, autoscan and ifnames programs now recognize the two
+ preprocessor directives, which were introduced in C23 and C++23.
+
 * Noteworthy changes in release 2.72 (2023-12-22) [release]
 
 ** Backward incompatibilities

bin/autom4te.in

@@ -636,7 +636,7 @@ sub handle_output ($$)
  # Don't complain in comments. Well, until we have something
  # better, don't consider '#include' etc. are comments.
  s/\#.*//
- unless /^\#\s*(if|include|endif|ifdef|ifndef|define)\b/;
+ unless /^\#\s*(include|endif|(el)?if(n?def)?|define)\b/;
  foreach (split (/\W+/))
  {
  $prohibited{$_} = $.
@@ -673,7 +673,7 @@ sub handle_output ($$)
  # Don't complain in comments. Well, until we have something
  # better, don't consider '#include' etc. to be comments.
  s/\#.*//
- unless /^\#(if|include|endif|ifdef|ifndef|define)\b/;
+ unless /^\#(include|endif|(el)?if(n?def)?|define)\b/;
 
  # Complain once per word, but possibly several times per line.
  while (/$prohibited/)

bin/autoscan.in

@@ -276,7 +276,7 @@ sub scan_c_file ($)
  {
  used ('header', $1);
  }
- if (s/^(if|ifdef|ifndef|elif)\s+//)
+ if (s/^(el)?if(n?def)?\s+//)
  {
  foreach my $word (split (/\W+/))
  {

bin/ifnames.in

@@ -55,9 +55,10 @@ $help = "Usage: $0 [OPTION]... [FILE]...
 
 Scan all of the C source FILES (or the standard input, if none are
 given) and write to the standard output a sorted list of all the
-identifiers that appear in those files in '#if', '#elif', '#ifdef', or
-'#ifndef' directives. Print each identifier on a line, followed by a
-space-separated list of the files in which that identifier occurs.
+identifiers that appear in those files in '#if', '#ifdef', '#ifndef',
+'#elif', '#elifdef', or '#elifndef' directives. Print each identifier
+on a line, followed by a space-separated list of the files in which
+that identifier occurs.
 
  -h, --help print this help, then exit
  -V, --version print version number, then exit
@@ -109,7 +110,7 @@ sub scan_file ($)
  while (s/\\$//);
 
  # Preprocessor directives.
- if (s/^\s*\#\s*(if|ifdef|ifndef|elif)\s+//)
+ if (s/^\s*\#\s*(el)?if(n?def)?\s+//)
  {
  # Remove comments. Not perfect, but close enough.
  s(/\*.*?\*/)();

doc/autoconf.texi

@@ -1478,8 +1478,9 @@ Invocation}).
 @command{ifnames} scans all of the C source files named on the command line
 (or the standard input, if none are given) and writes to the standard
 output a sorted list of all the identifiers that appear in those files
-in @code{#if}, @code{#elif}, @code{#ifdef}, or @code{#ifndef}
-directives. It prints each identifier on a line, followed by a
+in @code{#if}, @code{#ifdef}, @code{#ifndef}, @code{#elif},
+@code{#elifdef}, or @code{#elifndef} directives.
+It prints each identifier on a line, followed by a
 space-separated list of the files in which that identifier occurs.
 
 @noindent

tests/tools.at

@@ -732,6 +732,8 @@ LINE2
 
 AT_DATA([iftest2.c],
 [[#ifdef IFTEST2
+#elifdef IFTEST3
+#elifndef IFTEST4
 #if VAL1
 ]])
 
@@ -741,6 +743,8 @@ DEF2 iftest1.c
 DEF3 iftest1.c
 DEF4 iftest1.c
 IFTEST2 iftest2.c
+IFTEST3 iftest2.c
+IFTEST4 iftest2.c
 LINE1 iftest1.c
 LINE2 iftest1.c
 SPACES iftest1.c