[gnea/grbl Issue#419] Compile problems when using ATmega644p

Hi,
I try to compile GRBL for an ATMega644p with following settings in Makefile:

DEVICE ?= atmega644p
CLOCK = 20000000

This results in following problems during compilation:

In file included from grbl/grbl.h:31:0,
from grbl/serial.c:22:
grbl/serial.c: In Funktion »USARTUDREvect«:
grbl/cpumap.h:34:25: Warnung: »USARTUDREvect« appears to be a misspelled signal handler, missing _vector prefix [-Wmisspelled-isr]
#define SERIALUDRE USARTUDRE_vect
grbl/serial.c:107:5: Anmerkung: bei Substitution des Makros »SERIAL_UDRE«
ISR(SERIAL_UDRE)
grbl/serial.c: In Funktion »USARTRXvect«:
grbl/cpumap.h:33:25: Warnung: »USARTRXvect« appears to be a misspelled signal handler, missing _vector prefix [-Wmisspelled-isr]
#define SERIALRX USARTRX_vect
grbl/serial.c:143:5: Anmerkung: bei Substitution des Makros »SERIAL_RX«
ISR(SER

Any idea what this means? Is this a problem?

Thanks!