Explicitly specify a base name for the export header generation.

The library target is renamed from "Urho3D" to "Urho3D_lib" as it clashes with "Urho3D" target for script host application. The library target's output name is then adjusted back "Urho3D" in the macro to generate "libUrho3D.a" or "libUrho3D.so" or "Urho3D.dll", etc. However, it seems that generate_export_header() macro provided by CMake does not taken into consideration of the adjusted target's output name property. Fortunately, it accepts a BASE_NAME argument to rectify the problem cleanly.