helpers: Move static_assert definition to shared

Collect the fallback definitions of static_assert() from desktop-shell
and the test shell, and move them to helpers.h. This allows code
throughout the tree to use static_assert() for build-time assertions,
where it is supported by the compiler.

As GCC goes out of its way to only add static_assert() when C11 has been
explicitly requested - which we don't do - make sure to use the more
widely available _Static_assert() if that is provided.

This will be used in future patches to ensure two array lengths don't go
out of sync.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

desktop-shell/shell.c

@@ -47,10 +47,6 @@
 #define DEFAULT_NUM_WORKSPACES 1
 #define DEFAULT_WORKSPACE_CHANGE_ANIMATION_LENGTH 200
 
-#ifndef static_assert
-#define static_assert(cond, msg)
-#endif
-
 struct focus_state {
 	struct desktop_shell *shell;
 	struct weston_seat *seat;