Hello,
When building the AVR_ATMega323_WinAVR Demo with "make", poisoned signal errors raise.
Specifically, these errors are the following:
serial/serial.c:188:9: error: attempt to use poisoned "SIG_UART_RECV" SIGNAL( SIG_UART_RECV ) ^ serial/serial.c:207:9: error: attempt to use poisoned "SIG_UART_DATA" SIGNAL( SIG_UART_DATA )
To solve the problem, I simply changed both signal names as this documentation suggests.
That is, SIG_UART_RECV is replaced by USART_RXC_vect and SIG_UART_DATA by USART_UDRE_vect.

I'm new to these kind of projects, so I hope this solves the problem successfully.