build: enable use of versioned SONAME

[vincentbernat]

Most libraries are using a versioned SONAME to track API changes. When
they don't, we usually use a SOVERSION of 0. When willing to commit to
some API stability, SOVERSION and VERSION should be updated to more
adequate values (VERSION could match the library version, assuming it
follows the major.minor.patch convention and SOVERSION could be
major.minor since CMake doesn't support libtool versioning scheme with
age).

With this change CMake will install the library as
libcbor.so.0.0.0. libcbor.so.0 will be a symlink to the library for
other applications, to be found with the dynamic linker. libcbor.so will
be a symlink to libcbor.so.0 for use by the linker (ld).

[vincentbernat]

For SOVERSION, since CMake doesn't allow to use libtool convention, some big projects just use a single integer they increment each time the API changes. This number is not correlated to the version number. This would be something like that (I just use a big number to show the concept):

# Increment each time the public API is changed
set(CBOR_API_VERSION 17)
set_target_properties(cbor_shared PROPERTIES OUTPUT_NAME cbor SOVERSION ${CBOR_API_VERSION})
if(WINDOWS)
  set_target_properties(cbor_shared PROPERTIES VERSION ${CBOR_VERSION})
endif()

The other way is to have something like this:

set_target_properties(cbor_shared PROPERTIES OUTPUT_NAME cbor 
     SOVERSION ${CBOR_VERSION_MAJOR}.${CBOR_VERSION_MINOR}
     VERSION ${CBOR_VERSION})

[PJK]

Thanks, @vincentbernat, I learned something new today :)

This looks good enough to start with. I'm only vaguely familiar with libtool's conventions and I haven't really taken it into account since I find it quite difficult to work with when using build system other than autotools (correct me if I'm wrong).

Is there any significant drawback to using the

set_target_properties(cbor_shared PROPERTIES OUTPUT_NAME cbor 
     SOVERSION ${CBOR_VERSION_MAJOR}.${CBOR_VERSION_MINOR}
     VERSION ${CBOR_VERSION})

approach? It seems like it would fit in nicely with semver

[vincentbernat]

As long as you think to at least increase the minor version when changing API, I think this is fine.