Added warning about libtool

If libtool isn't installed ./configure results in some generic errors that don't explain the source of the problem. Added a check in autogen.sh to notify the user if they don't have Libtool.
I agree on the idea.

I think strictly spoken what it needs is 'libtoolize' (which either can be in the path or can be overridden with LIBTOOLIZE). See also the statements after it.

@theuni any comments? do you know is there an argument on autoreconf to automagically fail if we can't libtoolize?