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.

m4/.gitignore

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