Allow clients to override WL_EXPORT
In Chromium, we're building part of wayland-client with a config that requires wayland symbols to be hidden. There's no way to override the WL_EXPORT macro, so we're using a hack by passing -D"visibility(x)=visibility("hidden")" to the compiler. It would be ideal if we could override WL_EXPORT in a supported way. This can be done by making the following change in wl-util.h:
/** Visibility attribute */
#ifndef WL_EXPORT
#if defined(__GNUC__) && __GNUC__ >= 4
#define WL_EXPORT __attribute__ ((visibility("default")))
#else
#define WL_EXPORT
#endif
#endif