Wrap SoftwareSerial _SS_MAX_RX_BUFF define

To allow sketches to override value as per arduino#58.

Author: sandeepmistry

hardware/arduino/avr/libraries/SoftwareSerial/src/SoftwareSerial.h

@@ -39,7 +39,10 @@ The latest version of this library can always be found at
 * Definitions
 ******************************************************************************/
 
+#ifndef _SS_MAX_RX_BUFF
 #define _SS_MAX_RX_BUFF 64 // RX buffer size
+#endif
+
 #ifndef GCC_VERSION
 #define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
 #endif