fallback on lsof if otool not found in libedit detection
fallback on lsof if otool not found in libedit detection

This is necessary for Macs without XCode.
It also prevents a crash in the unlikely event that neither is found.
lsof detection is vulnerable to false positives if other (non-readline)
modules are linked against libedit, but this is also unlikely.

closes gh-524

ipython/ipython,ipython/ipython