Ok, here's the final optimized wiring_serial.c with output buffer that can be completely disabled by setting #define TX_BUFFER_SIZE 0
I had earlier pondered how this sort of #define trick could set the RX_BUFFER_SIZE also, since it's fairly large for most of the sketches that we see on the boards, but necessary for real workhorse applications that require constant serial attention.
The downside to this situation is that it actually depends on the LIBRARY to be rebuilt when the SKETCH uses it. It's no longer a shared library implementation, and a makefile might not establish that relationship. I would think you'd have to keep deleting the library .o files whenever you want to readjust the buffer size.
Am I wrong on this?