SOVERSION
What version number is this target.
For shared libraries VERSION and SOVERSION can be used to specify the build version and API version respectively. When building or installing appropriate symlinks are created if the platform supports symlinks and the linker supports so-names. If only one of both is specified the missing is assumed to have the same version number. SOVERSION is ignored if NO_SONAME property is set. For shared libraries and executables on Windows the VERSION attribute is parsed to extract a “major.minor” version number. These numbers are used as the image version of the binary.
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https://cmake.org/cmake/help/v3.5/prop_tgt/SOVERSION.html