Wayland: logging

The core libwayland libraries should not handle logging, only passing
the error messages to subscribed functions.
An application linked to libwayland-server or libwayland-client
will be able to set own functions (one per library) to handle error
messages.

Change in this series: make the wl_log return int, because
of compatibility with printf. It will return the number of bytes logged.

src/wayland-util.h

@@ -30,6 +30,7 @@ extern "C" {
 #include <math.h>
 #include <stddef.h>
 #include <inttypes.h>
+#include <stdarg.h>
 
 /* GCC visibility */
 #if defined(__GNUC__) && __GNUC__ >= 4
@@ -203,6 +204,8 @@ static inline wl_fixed_t wl_fixed_from_int(int i)
 	return i * 256;
 }
 
+typedef void (*wl_log_func_t)(const char *, va_list);
+
 #ifdef  __cplusplus
 }
 #endif