libreadline on the Mac is replaced by libedit

[DanGrayson]

See man editline for details. We should modify our configure script to detect its presence and use it, rather than building libreadline ourselves, or depending on brew to install readline for us. Maybe all we have to do is to call int el_set(EditLine *e, int op, ...); to get interrupts not handled, and everything else will work as is, because editline is a good impersonator of readline (except for not having rl_catch_signals`.)

Under Ubuntu it's provided in the package libedit-dev.