PRR and BOD registers in Attiny13A. How to tell AVR it is an attiny13A

The Attiny13A does have a PRR register that the Attiny13 doesnt have. The PRR register is defined in the “iotn13a.h” file that is called in the io.h file in folder: arduino-1.06/hardware/tools/avr/lib/avr/include/avr.
That file shows that the avr included with IDE 1.06 already knows the difference between an Attiny13 and an attiny13A in the following section
#elif defined (AVR_ATtiny13)

include <avr/iotn13.h>

#elif defined (AVR_ATtiny13A)

include <avr/iotn13a.h>

So, in order to use the iotn13a.h file rather than using the iotn13.h file I have to tell the compiler that it is an Attiny13A, not an Attiny13.

As far as I know I can do that in the boards.txt, so I copied a section for the Attiny13 and renamed the build.mcu section attiny13A, like this
also added an Attiny13 section in the avrdude.conf file

However, when I then compiled a file, i got an error message that it was an unknown mcu.

I know I still can use the new registers in the Attiny13A simply by adding:
#define PRR _SFR_IO8(0x25)
#define PRADC 0
#define PRTIM0 1
#define BODCR _SFR_IO8(0x30)
#define BPDSE 0
#define BPDS 1
to the program file, but that is a workaround.
As the avr seems to know the Attiny13, how do I tell the AVR it is an attiny13A and not an attiny13?