Fix for bug #4425:

- Assert was causing blender to crash on non-manifold meshes with a decimation
  modifier. Even in a debug build, there's no reason to crash blender then,
  because the modifier handles this ok and displays a warning message.

  Part of the problem is NDEBUG not being set in scons/linux for release builds,
  but always enabled for makefiles even in debug builds.

intern/decimation/intern/LOD_MeshPrimitives.cpp

@@ -199,8 +199,6 @@ SwapFace(
 	if (old_f == m_faces[1]) {
 		m_faces[1] = new_f;
 	} else {
-		MT_assert(false);
-
 		LOD_MeshException e(LOD_MeshException::e_search_error);
 		throw(e);	
 	}