SIMBODY_SONAME_VERSION not updated after rerunning CMake.

I don't think this needs to be cached. If it is cached, if I check out a different branch of Simbody, using the same CMake build directory, then this variable is not updated. Would a user ever need to modify this themselves?

```
SET(SIMBODY_SONAME_VERSION "${SIMBODY_MAJOR_VERSION}.${SIMBODY_MINOR_VERSION}"
    CACHE STRING
    "Soname version to use when generating shared libs")
```