c++: fix old cast warnings

We still have a few old C casts in lalr1.cc, let's get rid of them.
Reported by Frank Heckenbach.

Actually, let's monitor all our casts using easy to grep macros.
Let's use these macros to use the C++ standard casts when we are in
C++.

* data/skeletons/c.m4 (b4_cast_define): New.
* data/skeletons/glr.c, data/skeletons/glr.cc,
* data/skeletons/lalr1.cc, data/skeletons/stack.hh,
* data/skeletons/yacc.c:
Use it and/or its casts.

* tests/actions.at, tests/cxx-type.at,
* tests/glr-regression.at, tests/headers.at, tests/torture.at,
* tests/types.at:
Use YY_CAST instead of C casts.

* configure.ac (warn_cxx): Add -Wold-style-cast.
* doc/bison.texi: Disable it.

configure.ac

@@ -107,7 +107,8 @@ if test "$enable_gcc_warnings" = yes; then
     -Wpointer-arith -Wshadow
     -Wwrite-strings'
   warn_c='-Wbad-function-cast -Wstrict-prototypes'
-  warn_cxx='-Wextra-semi -Wnoexcept -Wundefined-func-template -Wweak-vtables'
+  warn_cxx='-Wextra-semi -Wnoexcept -Wold-style-cast -Wundefined-func-template
+    -Wweak-vtables'
   # Warnings for the test suite only.
   #
   # -fno-color-diagnostics: Clang's use of colors in the error