warnings: don't use _Noreturn with G++ 4.7 in C++98 mode

The timevar and bitset modules now use the c99 module which causes
$CXX to now include -std=gnu++11 when possible.  Unfortunately, G++
4.7 does not implement [[noreturn]] in C++11 mode, so our tests of
glr.cc (which uses _Noreturn) fail with

    input.cc:954:1: error: expected unqualified-id before '[' token

right before [[noreturn]].  4.8 works fine.

* data/skeletons/c.m4 (b4_attribute_define): Do not use [[noreturn]]
with GCC 4.7.

README-hacking

@@ -288,6 +288,24 @@ release:
 - Test with a very recent version of GCC for both C and C++.  Testing with
   older versions that are still in use is nice too.
 
+** gnulib
+To run tests on gnulib components (e.g., on bitset):
+
+    cd gnulib
+    ./gnulib-tool --test bitset-tests
+
+possibly within a specified environment:
+
+    CC='gcc-mp-8 -fsanitize=undefined' ./gnulib-tool --test bitset-tests
+
+To be able to run the tests several times, and to use symlinks instead of
+copies so that one can update the origin gnulib directory and immediately
+re-run the tests, run:
+
+    ./gnulib-tool --symlink --create-test --dir=/tmp/gnutest bitset-tests
+    cd /tmp/gnutest
+    ./configure CC='gcc-mp-8 -fsanitize=undefined'
+    make check
 
 * Release Procedure
 This section needs to be updated to take into account features from gnulib.

data/skeletons/c.m4

@@ -229,18 +229,22 @@ m4_define([b4_attribute_define],
 #endif
 
 ]m4_bmatch([$1], [\bnoreturn\b], [[/* The _Noreturn keyword of C11.  */
-#if ! defined _Noreturn
-# if defined __cplusplus && 201103L <= __cplusplus
+]dnl This is an exact copy of lib/_Noreturn.h.
+[#ifndef _Noreturn
+# if (defined __cplusplus \
+      && ((201103 <= __cplusplus && !(__GNUC__ == 4 && __GNUC_MINOR__ == 7)) \
+          || (defined _MSC_VER && 1900 <= _MSC_VER)))
 #  define _Noreturn [[noreturn]]
-# elif !(defined __STDC_VERSION__ && 201112 <= __STDC_VERSION__)
-#  if (3 <= __GNUC__ || (__GNUC__ == 2 && 8 <= __GNUC_MINOR__) \
-       || 0x5110 <= __SUNPRO_C)
-#   define _Noreturn __attribute__ ((__noreturn__))
-#  elif defined _MSC_VER && 1200 <= _MSC_VER
-#   define _Noreturn __declspec (noreturn)
-#  else
-#   define _Noreturn
-#  endif
+# elif ((!defined __cplusplus || defined __clang__) \
+        && (201112 <= (defined __STDC_VERSION__ ? __STDC_VERSION__ : 0)  \
+            || 4 < __GNUC__ + (7 <= __GNUC_MINOR__)))
+   /* _Noreturn works as-is.  */
+# elif 2 < __GNUC__ + (8 <= __GNUC_MINOR__) || 0x5110 <= __SUNPRO_C
+#  define _Noreturn __attribute__ ((__noreturn__))
+# elif 1200 <= (defined _MSC_VER ? _MSC_VER : 0)
+#  define _Noreturn __declspec (noreturn)
+# else
+#  define _Noreturn
 # endif
 #endif
 

m4/.gitignore

@@ -198,3 +198,4 @@
 /libtextstyle.m4
 /rename.m4
 /rmdir.m4
+/std-gnu11.m4