adding -fsigned-char to CFLAGS

[Jeff Sparkes]

Is there a way to add -fsigned-char to the gcc flags on systems with
unsigned char types?
I know I can detect that with autoconf, but I don't want to change the code
I'm compiling.
I've started compiling on AIX, but find that I need to edit the Makefile, or
always
pass -fsigned-char even on platforms that don't need it.  The latter works,
but seems ugly.