Fix iOS build with CMake 3.8
The iOS build (i.e. specifying -DVTK_IOS_BUILD:BOOL=ON) failed with an error that the vtkEncodeString utility is not found. The failure was first introduced in commit 894acce1.
This commit changes the logic to import VTKCompileTools when CMAKE_CROSSCOMPILING_EMULATOR is not defined even if CMake fully supports it.