borrowing WString.h from master

Author: Federico Fissore

hardware/arduino/avr/cores/arduino/WString.h

@@ -147,6 +147,7 @@ class String
 	void getBytes(unsigned char *buf, unsigned int bufsize, unsigned int index=0) const;
 	void toCharArray(char *buf, unsigned int bufsize, unsigned int index=0) const
 		{getBytes((unsigned char *)buf, bufsize, index);}
+	const char * c_str() const { return buffer; }
 
 	// search
 	int indexOf( char ch ) const;