Marlin error Rx_buffer not declared in scope
The line defining SERIAL_PORT in MarlinSerial.h should read:

#define SERIAL_PORT 0


To [mis-]quote this answer from RepRap:'rx_buffer' was not declared in this scope (the emphasis is mine):


#define SERIAL_PORT 4

  
  This is the serial port on the controller, not the serial port on your
  computer. 0 through 3 are valid options on a controller with a atmel
  2560. 
  
  Unless you are doing something like serial over bluetooth or
  deliberately not using the USB port you should not change this from 0


The same error is reported on the Arduino forums, 'rx_buffer' was not declared in this scope:


  Have redone over 5 times and always getting " 'rx_buffer' was not
  declared in this scope ".
  
  Now I tried Factory marlin and same "
  'rx_buffer' was not declared in this scope " . Dont know what to do - I'm
  so lost.
  
  ...

#define SERIAL_PORT 4     //before was 0

  
  Nothing else