Why am I getting the error: "`WDRT' was not declared in this scope" trying to turn of the WDT on my AVR?
There is a typo (twice!) in the datasheet.

The relevant bit is called WDRF in the header file...

#define MCUSR   _SFR_IO8(0x34)
#define PORF    0
#define EXTRF   1
#define BORF    2
#define WDRF    3


...which is consistent with the names of the other flags on that register. 

The code to clear the Watchdog System Reset Flag should read...

MCUSR &amp;= ~_BV(WDRF);