[PATCH] Rework autocrap configuration for libedit/libreadline

[Nils Goroll]

After an embarrassingly failing initial attempt, here's a patch to (hopefully)
fix #1555 and other build issues regarding libedit vs. libreadline.

@fgs: As discussed on irc, would be cool if you could check this on the build farm

@all: would appreciate more test on different configs, especially more exotic
ones like bsd, macos and the like

Nils