“__builtin_avr_delay_cycles expects a compile time integer constant”
I looked that up in avr_libc documentation for<util/delay.h> and found this:
"The implementation of _delay_ms() based on __builtin_avr_delay_cycles() is not backward compatible with older implementations.”

Adding the macro DELAY_BACKWARD_COMPATIBLE fixed it.

When using the UART async mode example, I'm missing the first of all char in serial monitor.
The normal UART example (without ring buffer) works fine.

A license on this github project would help readers of your blog know where they stand with following and using the tutorials/demos.

Error message #1: “return type of 'main' is not 'int’”
In main.c, I changed uint8_t to int and the problem went away.

include <avr/io.h>

include <stdio.h>

ifndef BAUD

define BAUD 9600

endif

include <util/setbaud.h>

/* http://www.cs.mun.ca/~rod/Winter2007/4723/notes/serial/serial.html */
int uart_putchar(char c, FILE *stream); // <=======//// RIGHT HERE
int uart_getchar(FILE *stream); // <=======//// AND HERE

// ////REQUIRED BY ======v===================v
FILE uart_output = FDEV_SETUP_STREAM(uart_putchar, NULL, _FDEV_SETUP_WRITE);
FILE uart_input = FDEV_SETUP_STREAM(NULL,uart_getchar, _FDEV_SETUP_READ);

void uart_init(void) {
UBRR0H = UBRRH_VALUE;
UBRR0L = UBRRL_VALUE;

UCSR0C = _BV(UCSZ01) | _BV(UCSZ00); /* 8-bit data */ 
UCSR0B = _BV(RXEN0) | _BV(TXEN0);   /* Enable RX and TX */    

}

int uart_putchar(char c, FILE *stream) {
if (c == '\n') {
uart_putchar('\r', stream);
}
loop_until_bit_is_set(UCSR0A, UDRE0);
UDR0 = c;
return 0;
}

int uart_getchar(FILE *stream) {
loop_until_bit_is_set(UCSR0A, RXC0);
return UDR0;
}