Import latest m4/m4.m4.

* submodules/autoconf: Update to autoconf 2.64.
* configure.ac (M4_GNU_OPTION): New define.
* src/output.c (output_skeleton): Use it to resolve FIXME.
* NEWS: Mention this.

Signed-off-by: Eric Blake <ebb9@byu.net>
(cherry picked from commit b9ad39c1fb)

ChangeLog

@@ -1,3 +1,11 @@
+2009-08-20  Eric Blake  <ebb9@byu.net>
+
+	Import latest m4/m4.m4.
+	* submodules/autoconf: Update to autoconf 2.64.
+	* configure.ac (M4_GNU_OPTION): New define.
+	* src/output.c (output_skeleton): Use it to resolve FIXME.
+	* NEWS: Mention this.
+
 2009-08-19  Joel E. Denny  <jdenny@clemson.edu>
 
 	Fix complaints about escape sequences.

NEWS

@@ -113,6 +113,8 @@ Bison News
 
 * Changes in version 2.4.2 (????-??-??):
 
+** Detection of GNU M4 1.4.6 or newer during configure is improved.
+
 ** %code is now a permanent feature.
 
   A traditional Yacc prologue directive is written in the form:

configure.ac

@@ -94,6 +94,8 @@ AC_PROG_YACC
 AC_PROG_RANLIB
 AC_PROG_GNU_M4
 AC_DEFINE_UNQUOTED([M4], ["$M4"], [Define to the GNU M4 executable name.])
+AC_DEFINE_UNQUOTED([M4_GNU_OPTION], ["$M4_GNU"], [Define to "-g" if GNU M4
+supports -g, otherwise to "".])
 AM_MISSING_PROG([HELP2MAN], [help2man])
 AC_PATH_PROG([XSLTPROC], [xsltproc])
 AC_SUBST([XSLTPROC])

src/output.c

@@ -491,7 +491,7 @@ output_skeleton (void)
 {
   FILE *in;
   int filter_fd[2];
-  char const *argv[9];
+  char const *argv[10];
   pid_t pid;
 
   /* Compute the names of the package data dir and skeleton files.  */
@@ -542,6 +542,19 @@ output_skeleton (void)
   {
     int i = 0;
     argv[i++] = m4;
+
+    /* When POSIXLY_CORRECT is set, GNU M4 1.6 and later disable GNU
+       extensions, which Bison's skeletons depend on.  With older M4,
+       it has no effect.  M4 1.4.12 added a -g/--gnu command-line
+       option to make it explicit that a program wants GNU M4
+       extensions even when POSIXLY_CORRECT is set.
+
+       See the thread starting at
+       <http://lists.gnu.org/archive/html/bug-bison/2008-07/msg00000.html>
+       for details.  */
+    if (*M4_GNU_OPTION)
+      argv[i++] = M4_GNU_OPTION;
+
     argv[i++] = "-I";
     argv[i++] = pkgdatadir;
     if (trace_flag & trace_m4)
@@ -552,25 +565,7 @@ output_skeleton (void)
     argv[i++] = full_skeleton;
     argv[i++] = NULL;
   }
-  /* When POSIXLY_CORRECT is set, some future versions of GNU M4 (most likely
-     2.0) may drop some of the GNU extensions that Bison's skeletons depend
-     upon.  So that the next release of Bison is forward compatible with those
-     future versions of GNU M4, we unset POSIXLY_CORRECT here.
 
-     FIXME: A user might set POSIXLY_CORRECT to affect processes run from
-     macros like m4_syscmd in a custom skeleton.  For now, Bison makes no
-     promises about the behavior of custom skeletons, so this scenario is not a
-     concern.  However, we eventually want to eliminate this shortcoming.  The
-     next release of GNU M4 (1.4.12 or 1.6) will accept the -g command-line
-     option as a no-op, and later releases will accept it to indicate that
-     POSIXLY_CORRECT should be ignored.  Once the GNU M4 versions that accept
-     -g are pervasive, Bison should use -g instead of unsetting
-     POSIXLY_CORRECT.
-
-     See the thread starting at
-     <http://lists.gnu.org/archive/html/bug-bison/2008-07/msg00000.html>
-     for details.  */
-  unsetenv ("POSIXLY_CORRECT");
   init_subpipe ();
   pid = create_subpipe (argv, filter_fd);
   free (full_m4sugar);