@stahta01:
Your trick using STRINGIFY doesn't work with avr-gcc. This probably depends on the behavior of the c preprocessor and might be implementation dependent.
@oBFusCATed:
I tested several versions with double and triple " with and without preceding \ – nothing works. It seems to be impossible to define a double quoted -D macro. Also using ' to protect " doesn't help.
The only way I found, to do the trick is adding the folder which contains the header file to the -I options and define the macro as -DUART_CONFIG_H=<uart_config.h>, but that's not really conforming to the rules.