tests: skip C++ tests that are too demanding for some compilers

Some tests now fail when compiled with G++ 4.3 or 4.4 on MacPorts.

* tests/local.at (AT_SKIP_IF_EXCEPTION_SUPPORT_IS_POOR): New.
* tests/c++.at (Exception safety): Use it.
style: use /* ... */ comments

* src/complain.c: Here.
gnulib: update
skeletons: update the handling of compiler attributes

* data/c.m4 (b4_attribute_define): Instead of defining __attribute__,
define YY_ATTRIBUTE conditionally.
(YY_ATTRIBUTE_PURE, YY_ATTRIBUTE_UNUSED, _Noreturn): New.
Use them.
* data/glr.c: Use them.
c++: use __attribute__((__pure__)) to avoid warnings

Building C++ parsers with -Wsuggest-attribute=const and
-Wsuggest-attribute=noreturn triggers warning in generated code.

* data/lalr1.cc: Call b4_attribute_define.
(debug_stream, debug_level): Flag as pure.
* tests/headers.at (Several parsers): There are now more YY macros
that "leak".
build: restore maintainer-push-check

* tests/local.mk: here.
build: use Automake 1.14's non-recursive Makefile features

* configure.ac: Require Automake 1.14.
* examples/calc++/local.mk, examples/local.mk, examples/mfcalc/local.mk,
* examples/rpcalc/local.mk, tests/local.mk: Use %D% and %C%.