Converting C project to C++ project is not supported yet. However i found a trick to manually do it.
In the project directory, you should find a “.project” file. Add “org.eclipse.cdt.core.ccnature” under the “natures” parent element, then save the changes.
The project has to be refreshed (or close then re-open).
Under the project properties, you should find the G++ settings :
Hope it help you.