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 did this after trying to install this on my ubuntu - it crashed for some reason, looked it up, installed libtool and now it works. Would it be possible to include libtool in-tree? Seems like it's confused a few people as it was never there before