configure.ac: be more helpful on the libtool versioning
configure.ac: be more helpful on the libtool versioning

The versioning by libtool is sometimes a bit confusing and seems
complex. But it's really important to do a proper versioning of the
library.
Rephrase the part about how to check the needed versioning to be a bit
more informative how to do that.

Some more in deep explanation about the meaning of the libtool version
number can be found on:

* https://autotools.io/libtool/version.html
* http://www.informit.com/articles/article.aspx?p=19553
* https://developer.gnome.org/programming-guidelines/stable/versioning.html.en

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap