[PATCH] Fix GiNaC library version information.

[PATCH] Fix GiNaC library version information.

The libtool naming scheme cannot guarantee that on all systems, the numbering
is consecutive. It only guarantees that it is increasing. This doesn't matter,
though: there is not incurred cost for numbers that are omitted, except for
shrinking the available space of leftover numbers. Not something we need to
worry about yet.  ;-)  On the other hand, tricks which we were using to make
version numbers consecutive on GNU/Linux break versioning on other OSes.