Cazfi's version comparison library cvercmp

Homepage: https://www.cazfi.net/other/cvercmp.html

As the copyright holder, cazfi has granted freeciv project right to
use the provided version under GPLv2+.

Freeciv's current version is about the same as upstream release 1.0.5,
with some modifications, including:
- fc_strcasecmp() used instead of strcasecmp()
