Don't add so version to modules on macOS

Fixes error during linking, as loadable modules can't have versions
on macOS:

c++: error: invalid argument '-compatibility_version 164.0.0' only allowed with '-dynamiclib'

cmake/osgearth-macros.cmake

@@ -48,7 +48,7 @@ macro(add_osgearth_plugin)
     set_target_properties(${MY_TARGET} PROPERTIES PREFIX "")
 
     # soversions - append SO version to shared object files on unix (e.g., osgearth.so.123)
-    if (OSGEARTH_SONAMES)
+    if (NOT APPLE AND OSGEARTH_SONAMES)
         set_target_properties(${MY_TARGET} PROPERTIES VERSION ${OSGEARTH_VERSION} SOVERSION ${OSGEARTH_SOVERSION})
     endif()