portability: fix several issues with M4 subprocess.

M4's output pipe was not being drained upon fatal errors during
scan_skel.  As a result, broken-pipe messages from M4 were seen
on at least AIX, HP-UX, Solaris, and RHEL4, and this caused a
failure in the test suite.  The problem was that, on platforms
where the default disposition for SIGPIPE is ignore instead of
terminate, M4 sometimes saw fwrite fail with errno=EPIPE and
then reported it.  However, there's some sort of race condition,
because the new test group occasionally succeeded.
Reported by Albert Chin at
<http://lists.gnu.org/archive/html/bug-bison/2010-02/msg00004.html>.

There were also problems with the test suite livelocking on
Tru64 5.1b.  Reported by Didier Godefroy at
<http://lists.gnu.org/archive/html/bug-bison/2009-05/msg00005.html>.
Switching to create_pipe_bidi suggested by Akim Demaille.

To attempt to solve both of these problems, switch to gnulib's
create_pipe_bidi and register M4 process as a slave.  Along the
way, clean up file name conflict handling, which was affected by
the broken-pipe problem before the switch.
* NEWS (2.4.2): Document.
* THANKS (Didier Godefroy): Add.
* bootstrap.conf (gnulib_modules): Add pipe.
* gnulib: Update to latest to make sure we have all the latest
fixes.
* lib/local.mk (lib_libbison_a_SOURCES): Remove subpipe.h and
subpipe.c.
* po/POTFILES.in (lib/subpipe.c): Remove.
* src/files.c (compute_output_file_names): Update invocations
of output_file_name_check.
(output_file_name_check): In the case that the grammar file
would be overwritten, use complain instead of fatal, but replace
the output file name with /dev/null.  Use the /dev/null solution
for the case of two conflicting output files as well because it
seems safer in case Bison one day tries to open both files at
the same time.
* src/files.h (output_file_name_check): Update prototype.
* src/output.c (output_skeleton): Use create_pipe_bidi and
wait_subprocess.  Assert that scan_skel completely drains the
pipe.
* src/scan-skel.l (at_directive_perform): Update
output_file_name_check invocation.
* tests/output.at (AT_CHECK_CONFLICTING_OUTPUT): Check that the
grammar file actually isn't overwritten.
(Conflicting output files: -o foo.y): Update expected output.
* tests/skeletons.at (Fatal errors but M4 continues producing
output): New test group.
(cherry picked from commit 22cc8d813e)

Conflicts:

	NEWS
	bootstrap.conf
	lib/.cvsignore
	lib/.gitignore
	lib/Makefile.am
	m4/.cvsignore
	m4/.gitignore
	src/output.c

src/files.c

@@ -319,21 +319,21 @@ compute_output_file_names (void)
     {
       if (! spec_graph_file)
 	spec_graph_file = concat2 (all_but_tab_ext, ".dot");
-      output_file_name_check (spec_graph_file);
+      output_file_name_check (&spec_graph_file);
     }
 
   if (xml_flag)
     {
       if (! spec_xml_file)
 	spec_xml_file = concat2 (all_but_tab_ext, ".xml");
-      output_file_name_check (spec_xml_file);
+      output_file_name_check (&spec_xml_file);
     }
 
   if (report_flag)
     {
       if (!spec_verbose_file)
         spec_verbose_file = concat2 (all_but_tab_ext, OUTPUT_EXT);
-      output_file_name_check (spec_verbose_file);
+      output_file_name_check (&spec_verbose_file);
     }
 
   free (all_but_tab_ext);
@@ -342,18 +342,37 @@ compute_output_file_names (void)
 }
 
 void
-output_file_name_check (char const *file_name)
+output_file_name_check (char **file_name)
 {
-  if (0 == strcmp (file_name, grammar_file))
-    fatal (_("refusing to overwrite the input file %s"), quote (file_name));
-  {
-    int i;
-    for (i = 0; i < file_names_count; i++)
-      if (0 == strcmp (file_names[i], file_name))
-        warn (_("conflicting outputs to file %s"), quote (file_name));
-  }
-  file_names = xnrealloc (file_names, ++file_names_count, sizeof *file_names);
-  file_names[file_names_count-1] = xstrdup (file_name);
+  bool conflict = false;
+  if (0 == strcmp (*file_name, grammar_file))
+    {
+      complain (_("refusing to overwrite the input file %s"),
+                quote (*file_name));
+      conflict = true;
+    }
+  else
+    {
+      int i;
+      for (i = 0; i < file_names_count; i++)
+        if (0 == strcmp (file_names[i], *file_name))
+          {
+            warn (_("conflicting outputs to file %s"),
+                  quote (*file_name));
+            conflict = true;
+          }
+    }
+  if (conflict)
+    {
+      free (*file_name);
+      *file_name = strdup ("/dev/null");
+    }
+  else
+    {
+      file_names = xnrealloc (file_names, ++file_names_count,
+                              sizeof *file_names);
+      file_names[file_names_count-1] = xstrdup (*file_name);
+    }
 }
 
 void