In which I PEBKAC so you don’t have to

#define LED_BUILTIN _BV(PORTB5)

int main(void) { DDRB |= LED_BUILTIN;

while (1)
{
    PORTB |= LED_BUILTIN;   // turn on led
    _delay_ms(1000);        // delay 1s

    PORTB &= ~LED_BUILTIN;  // turn off led
    _delay_ms(1000);        // delay 1s
}                                                

}