[binutils-gdb/gdb-7.12-branch] gdb: Fix C and C++03 builds

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 9bfe0298332782a9c082fb475bdf8eeeef8cf45e ***

Author: Pedro Alves <palves@redhat.com>
Branch: gdb-7.12-branch
Commit: 9bfe0298332782a9c082fb475bdf8eeeef8cf45e

gdb: Fix C and C++03 builds

The readline/sjlj-exceptions fix added an unconditional use of
noexcept, but that's only valid C++11, and 7.12 must build with C and
C++03 too.  Fix this by adding a GDB_EXCEPT macro that compiles away
to nothing in C, and to throw() in C++03, which I've confirmed fixes
the original issue just the same as noexcept, with GCC 7 + -std=gnu+03
+ sjlj-exceptions.

gdb/ChangeLog:
2016-12-20  Pedro Alves  <palves@redhat.com>

	PR gdb/20977
	* event-top.c (GDB_NOEXCEPT): Define.
	(gdb_rl_callback_read_char_wrapper_noexcept): Use GDB_NOEXCEPT
	instead of noexcept and use (void) instead of ().
	(gdb_rl_callback_handler): Use GDB_NOEXCEPT instead of noexcept.