Merge remote-tracking branch 'origin/maint'

* origin/maint:
  maint: update gnu-web-doc-update.
  maint: post-release administrivia
  version 2.6
  maint: prepare for release 2.6
  maint: post-release administrivia
  version 2.5.91
  maint: prepare NEWS.
  maint: fix spaces.
  tests: adjust to case where the C compiler is actually a C++ compiler
  tests: fix dependencies
  doc: fix Texinfo command
  maint: Valgrind on OS X.
  tests: be sure that backups are safe.
  maint: dead comment.
  tests: refactor for legibility.
  tests: refactor the bison invocations.
  maint: fix syntax-check ignore patterns.
  gnulib: update
  gnulib: update.
  gnulib: update

Conflicts:
	build-aux/Makefile.am
	cfg.mk
	tests/Makefile.am

tests/local.at

@@ -20,6 +20,15 @@
 m4_version_prereq([2.58])
 
 
+# m4_null_if(VAL, IF-TRUE, IF-FALSE)
+# ----------------------------------
+# If VAL evaluates to empty or 0, run IF-TRUE, otherwise IF-FALSE.
+m4_define([m4_null_if],
+[m4_case(m4_quote($1),
+         [0], [$2],
+         [],  [$2],
+         [$3])])
+
 ## ------------- ##
 ## Basic tests.  ##
 ## ------------- ##
@@ -44,6 +53,8 @@ m4_divert_text([PREPARE_TESTS],
 {
   for at_save_file in stderr experr expout
   do
+    test ! -f at-bison-check-$at_save_file.bak ||
+      as_fn_error 1 "fatal error: back-up on top of a back-up"
     test ! -f $at_save_file || mv $at_save_file at-bison-check-$at_save_file.bak
   done
 }
@@ -407,6 +418,9 @@ void
 
 # AT_BISON_CHECK(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
 # -------------------------------------------------
+# High-level routine that may call bison several times, under different
+# conditions.
+#
 # Check Bison by invoking `bison BISON_ARGS'.  BISON_ARGS should not contain
 # shell constructs (such as redirection or pipes) that would prevent
 # appending additional command-line arguments for bison. OTHER_AT_CHECK_ARGS
@@ -427,24 +441,30 @@ void
 #
 #   4. If stderr contains a warning, -Wnone and --warnings=none suppress it.
 m4_define([AT_BISON_CHECK],
-[m4_if(m4_quote($2), [0], [AT_BISON_CHECK_XML($@)],
-       m4_quote($2), [], [AT_BISON_CHECK_XML($@)])
+[m4_null_if([$2], [AT_BISON_CHECK_XML($@)])
 AT_BISON_CHECK_NO_XML($@)])
 
 m4_define([AT_BISON_WERROR_MSG],
           [[bison: warnings being treated as errors]])
 
-# AT_BISON_CHECK_NO_XML(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
-# --------------------------------------------------------
-# Same as AT_BISON_CHECK except don't perform XML/XSLT checks.  This is useful
-# when a tortured grammar's XML is known to be too large for xsltproc to
-# handle.
-m4_define([AT_BISON_CHECK_NO_XML],
-[AT_CHECK(m4_if(m4_quote($2), [0], [], m4_quote($2), [], [],
-                [AT_QUELL_VALGRIND ])[[bison ]]$@)
-m4_if(m4_bregexp([$4], [: warning: ]), [-1], [],
-      m4_quote(m4_if(m4_quote($2), [], [0], [$2])), [0], [[
-        # Defining POSIXLY_CORRECT causes bison to complain if options
+
+# AT_BISON_CHECK_(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
+# --------------------------------------------------
+# Low-level macro to run bison once.
+m4_define([AT_BISON_CHECK_],
+[AT_CHECK(AT_QUELL_VALGRIND[[ bison ]]$@)])
+
+
+# AT_BISON_CHECK_WARNINGS(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
+# ----------------------------------------------------------
+# Check that warnings (if some are expected) are correctly
+# turned into errors with -Werror, etc.
+m4_define([AT_BISON_CHECK_WARNINGS],
+[m4_if(m4_bregexp([$4], [: warning: ]), [-1], [],
+      [m4_null_if([$2], [AT_BISON_CHECK_WARNINGS_($@)])])])
+
+m4_define([AT_BISON_CHECK_WARNINGS_],
+[[# Defining POSIXLY_CORRECT causes bison to complain if options
         # are added after the grammar file name, so skip these checks
         # in that case.
         if test -z "${POSIXLY_CORRECT+set}"; then
@@ -454,8 +474,7 @@ m4_if(m4_bregexp([$4], [: warning: ]), [-1], [],
           ]AT_DATA([expout], [$3])[
 
           # Run with -Werror.
-          ]AT_CHECK(AT_QUELL_VALGRIND[[ bison ]$1[ -Werror]],
-                    [[1]], [expout], [stderr])[
+  ]AT_BISON_CHECK_([$1[ -Werror]], [[1]], [expout], [stderr])[
 
           # Build expected stderr up to and including the "warnings being
           # treated as errors" message.
@@ -489,22 +508,27 @@ m4_if(m4_bregexp([$4], [: warning: ]), [-1], [],
 
           # Now check --warnings=error.
           cp stderr experr
-          ]AT_CHECK(AT_QUELL_VALGRIND[[ bison ]$1[ --warnings=error]],
-                    [[1]], [expout], [experr])[
+  ]AT_BISON_CHECK_([$1[ --warnings=error]], [[1]], [expout], [experr])[
 
           # Now check -Wnone and --warnings=none by making sure that
           # -Werror doesn't change the exit status when -Wnone or
           # --warnings=none is specified.
-          ]AT_CHECK(AT_QUELL_VALGRIND[[ bison ]$1[ -Wnone -Werror]],
-                    [[0]], [expout])[
-          ]AT_CHECK(AT_QUELL_VALGRIND[[ bison ]$1[ --warnings=none \
-                    -Werror]], [[0]], [expout])[
+  ]AT_BISON_CHECK_([$1[ -Wnone -Werror]], [[0]], [expout])[
+  ]AT_BISON_CHECK_([$1[ --warnings=none -Werror]], [[0]], [expout])[
 
           ]AT_RESTORE_SPECIAL_FILES[
-        fi
-      ]])
+fi]dnl
 ])
 
+# AT_BISON_CHECK_NO_XML(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
+# --------------------------------------------------------
+# Same as AT_BISON_CHECK except don't perform XML/XSLT checks.  This is useful
+# when a tortured grammar's XML is known to be too large for xsltproc to
+# handle.
+m4_define([AT_BISON_CHECK_NO_XML],
+[AT_CHECK(m4_null_if([$2], [], [AT_QUELL_VALGRIND ])[[bison ]]$@)
+AT_BISON_CHECK_WARNINGS($@)])
+
 # AT_BISON_CHECK_XML(BISON_ARGS, [OTHER_AT_CHECK_ARGS])
 # -----------------------------------------------------
 # Run AT_BISON_CHECK's XML/XSLT checks if $BISON_TEST_XML=1 and $XSLTPROC is
@@ -521,10 +545,10 @@ m4_define([AT_BISON_CHECK_XML],
                             [--xml=[^][ ]*], [])])dnl
   # Don't combine these Bison invocations since we want to be sure that
   # --report=all isn't required to get the full XML file.
-  AT_CHECK([[bison --report=all --report-file=xml-tests/test.output \
+  AT_BISON_CHECK_([[--report=all --report-file=xml-tests/test.output \
              --graph=xml-tests/test.dot ]]AT_BISON_ARGS,
            [[0]], [ignore], [ignore])
-  AT_CHECK([[bison --xml=xml-tests/test.xml ]]AT_BISON_ARGS,
+  AT_BISON_CHECK_([[--xml=xml-tests/test.xml ]]AT_BISON_ARGS,
            [[0]], [ignore], [ignore])
   m4_popdef([AT_BISON_ARGS])dnl
   [cp xml-tests/test.output expout]
@@ -553,6 +577,8 @@ m4_define([AT_BISON_CHECK_XML],
 m4_define([AT_QUELL_VALGRIND],
 [[[VALGRIND_OPTS="$VALGRIND_OPTS --leak-check=summary --show-reachable=no"; export VALGRIND_OPTS;]]])
 
+
+
 ## ------------------------ ##
 ## Compiling C, C++ Files.  ##
 ## ------------------------ ##