Fix SOVERSION in Linux

[Iniesta8]

In Linux (and BSD) the SOVERSION only consists of the major version number. The correct symbolic links have to be

libminizip.so -> libminizip.so.3
libminizip.so.3 -> libminizip.so.3.0.2
libminizip.so.3.0.2

and the SONAME of libminizip.so.3.0.2 is libminizip.so.3. By this change the symbolic links are created correctly.

On other platforms, such as Windows and macOS, the SOVERSION may well consist of <major-minor>. That's why I added the if-else statement.

See also: https://cmake.org/cmake/help/latest/prop_tgt/SOVERSION.html

[nmoinvaz]

I'm okay if it just says "3" with no if statement.

[Iniesta8]

update, without platform differentiation.