Fix separate library VERSION and SOVERSION variables.

src/CMakeLists.txt uses earch variable for its respective property:
```
set_target_properties(${SIDX_LIB_NAME}
    PROPERTIES VERSION "${SIDX_LIB_VERSION}"
               SOVERSION "${SIDX_LIB_SOVERSION}"  )

set_target_properties(${SIDX_C_LIB_NAME}
    PROPERTIES VERSION "{SIDX_LIB_VERSION}"
               SOVERSION "${SIDX_LIB_SOVERSION}" )
```

This issues is a followup for #44