Add configure switch to define USE_XON_XOFF

[Ivan Kozlov]

Readline tests that macro and suppresses software flow control. This is quite 
handy: I want software flow control, just not *in bash*, where it is not useful 
and I would like C-s to do incremental search instead. With USE_XON_XOFF, it is 
inactive in bash and is restored before executing a command alongside other 
termios settings. It is probably possible to hack this with PROMPT_COMMAND/PS0, 
but clearly this is a much cleaner and readily available option. The only thing 
lacking is a configure switch.