Use av_printf_format to check the usage of printf style functions

This helps catching cases where the format string doesn't
match what is passed in, or injection bugs where user data
is passed in as format string.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/log.h

@@ -23,6 +23,7 @@
 
 #include <stdarg.h>
 #include "avutil.h"
+#include "attributes.h"
 
 /**
  * Describe the class of an AVClass context structure. That is an
@@ -129,11 +130,7 @@ typedef struct {
  * subsequent arguments are converted to output.
  * @see av_vlog
  */
-#ifdef __GNUC__
-void av_log(void *avcl, int level, const char *fmt, ...) __attribute__ ((__format__ (__printf__, 3, 4)));
-#else
-void av_log(void *avcl, int level, const char *fmt, ...);
-#endif
+void av_log(void *avcl, int level, const char *fmt, ...) av_printf_format(3, 4);
 
 void av_vlog(void *avcl, int level, const char *fmt, va_list);
 int av_log_get_level(void);