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.