integrate the editline build in the configure and make system.

At the moment, due to some remaining issues on MacOS X the default
is still off. To test it use the --enable-editline configure flag.
Once all issues are ironed out we'll turn it on by default.


git-svn-id: http://root.cern.ch/svn/root/trunk@30192 27541ba8-7e3a-0410-8455-c3a389f83636