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 don't see how this helps. @laanwj is correct on both counts. Besides that, it already exits with a descriptive message:
```sh
cory@cory-i7:~/dev/bitcoin(master)$ sudo mv /usr/bin/libtoolize /usr/bin/libtoolize.old
cory@cory-i7:~/dev/bitcoin(master)$ ./autogen.sh 
Can't exec "libtoolize": No such file or directory at /usr/bin/autoreconf line 195.
Use of uninitialized value in pattern match (m//) at /usr/bin/autoreconf line 195.
Can't exec "libtoolize": No such file or directory at /usr/share/autoconf/Autom4te/FileUtils.pm line 345, <GEN3> line 5.
autoreconf: failed to run libtoolize: No such file or directory
autoreconf: libtoolize is needed because this package uses Libtool
```