Use -version-number in libtool

-version-info uses CURRENT/REVISION/AGE format, which is buggy in BSD
family [1] and it is not the same as cmake MAJOR/MINOR/REVISION format.
Using -version-number to replace -version-info can solve both the
problems.

Closed #55

[1] https://lists.gnu.org/archive/html/bug-libtool/2011-05/msg00007.html