Warn when Serial.printf() format is wrong

Let GCC check the format string to Print::printf()

Author: earlephilhower

Diff for: api/Print.h

@@ -90,8 +90,8 @@ class Print
     size_t println(void);
 
     // EFP3 - Add printf() to make life so much easier...
-    size_t printf(const char *format, ...);
-    size_t printf_P(const char *format, ...);
+    size_t printf(const char *format, ...) __attribute__ ((format (printf, 2, 3)));
+    size_t printf_P(const char *format, ...) __attribute__ ((format (printf, 2, 3)));
 
     virtual void flush() { /* Empty implementation for backward compatibility */ }
 };