CMake: Build LuaJIT: use copy_if_different
CMake: Build LuaJIT: use copy_if_different

Use copy_if_different to move luajit products in the post build step to
avoid relinking of projects depending on sol2/luagit.
This is mostly relevant when building sol2 and luajit as subprojects.

ThePhD/sol2,ThePhD/sol2,ThePhD/sol2,ThePhD/sol2