c++: use nullptr for C++11.

C++11 introduces "nullptr" which plays the role of C's NULL, in
replacement of "0".  Fix the C++ skeletons to avoid warnings about
uses of "0" in place of "nullptr", and improve C skeletons to also use
this "nullptr" when compiled with a C++11 compiler.

* configure.ac: More C++ warnings.
* NEWS (2.5.1): Document this.
* data/c++.m4, data/c.m4 (b4_null_define): New.
(b4_null): Use YY_NULL instead of 0.
* data/glr.c, data/lalr1.cc, data/location.cc, data/yacc.c:
Call b4_null_define/b4_null where appropriate.
Use YY_NULL instead of NULL.
* data/location.cc (initialize): Accept a default argument,
YY_NULL.
* tests/actions.at, tests/calc.at: Adjust.

* data/glr.c, lib/libiberty.h, src/system.h (__attribute__):
Do not disable it when __STRICT_ANSI__ is defined, as, for
instance, it disables the __attribute__((unused)) which
protects us from some compiler warnings.
This was already done elsewhere in Bison, in 2001, see
4a0d893695.
* tests/regression.at: Adjust output.

data/yacc.c

@@ -267,6 +267,8 @@ m4_if(b4_prefix, [yy], [],
 /* Copy the first part of user declarations.  */
 ]b4_user_pre_prologue[
 
+]b4_null_define[
+
 /* Enabling traces.  */
 #ifndef YYDEBUG
 # define YYDEBUG ]b4_debug_flag[