cmake: Add support for CMAKE_CROSSCOMPILING_EMULATOR

This commit updates the build systems to allow cross-compiling VTK
using toolchain defining CMAKE_CROSSCOMPILING_EMULATOR.

It simplifies the cross-compilation removing the need to independently
compile the "VTKCompileTools".

See https://cmake.org/cmake/help/v3.8/variable/CMAKE_CROSSCOMPILING_EMULATOR.html#variable:CMAKE_CROSSCOMPILING_EMULATOR