Always enable -DOVM_FORCE_STATIC_CAST on MacOS (see OpenFlipper/OpenFlipper-Free!136)

Without this workaround, propvis is not usable for OVM meshes on MacOS.

CMakeLists.txt

 include (plugin)
 
+if(APPLE)
+    # actually we currently need this for any libc++ platform
+    # compare issue 136:
+    # https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/issues/136
+    add_definitions(-DOVM_FORCE_STATIC_CAST)
+endif()
+
 openflipper_plugin (INSTALLDATA Icons  
                     DIRS OpenMesh OpenVolumeMesh Widgets ScriptObjects
                     DEPS OpenMesh