Choosing the correct version number with libtool

[Hal Duston]

I have a project that I am using libtool to help manage.  In preparing
for my next release, I have a question regarding setting the correct
version number for the library.

In the manual, section 6.3 talks about how if any interfaces have been
added and/or changed and/or removed how to set the version number
appropriately to reflect this.  My question is what is an "interface"?
I have changed the environment variables from my last previous release,
(by consolidating several environment variables into one), which I am
assuming would count as "removed", but is that considered part of the
"interface"?  As far as the actual function api of the library is
concerned, I have neither added, changed nor removed any public
functions.

Thanks,
--
Hal Duston