use _Static_assert in QEMU_BUILD_BUG_ON

QEMU_BUILD_BUG_ON should use C11's _Static_assert, if the compiler supports it,
to provide more readable messages on failure.

We check for _Static_assert in configure, and set CONFIG_STATIC_ASSERT
accordingly. QEMU_BUILD_BUG_ON invokes _Static_assert if CONFIG_STATIC_ASSERT
is defined, and reverts to the old way otherwise.

That way, systems without C11 conforming compiler will still have the old
messages, as verified by intentionally breaking the configure check.

the following example output was generated by inverting the condition in
QEMU_BUILD_BUG_ON:

without _Static_assert:

> In file included from /qemu/include/qemu/osdep.h:36:0,
>                  from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function â€˜qmp_guest_exec_statusâ€™:
> /qemu/include/qemu/compiler.h:89:12: error: negative width in bit-field â€˜<anonymous>â€™
>      struct { \
>             ^
> /qemu/include/qemu/compiler.h:96:38: note: in expansion of macro  QEMU_BUILD_BUG_ON_STRUCTâ€™
>  #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
>                                       ^~~~~~~~~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro â€˜QEMU_BUILD_BUG_ONâ€™
>      QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
>      ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro â€˜atomic_load_acquireâ€™
>      atomic_load_acquire(ptr)
>      ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro â€˜atomic_mb_readâ€™
>      bool finished = atomic_mb_read(&gei->finished);
>                      ^~~~~~~~~~~~~~

with _Static_assert:

> In file included from /qemu/include/qemu/osdep.h:36:0,
>                  from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function â€˜qmp_guest_exec_statusâ€™:
> /qemu/include/qemu/compiler.h:94:30: error: static assertion failed: "not expecting: sizeof(*&gei->finished) > sizeof(void *)"
>  #define QEMU_BUILD_BUG_ON(x) _Static_assert((x), #x)
>                               ^
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro â€˜QEMU_BUILD_BUG_ONâ€™
>      QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *));   \
>      ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro â€˜atomic_load_acquireâ€™
>      atomic_load_acquire(ptr)
>      ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro â€˜atomic_mb_readâ€™
>      bool finished = atomic_mb_read(&gei->finished);
>                      ^~~~~~~~~~~~~~

Signed-off-by: Andreas Grapentin <andreas@grapentin.org>
Message-Id: <20170314165953.18506-1-andreas@grapentin.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
