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.
Eh, yes, that message seems to be as clear and in-your-face as error messages get.

I mean why would

    Libtool is Missing. Bitcoin Requires Libtool To Compile.

be better than

    autoreconf: libtoolize is needed because this package uses Libtool

Closing this...