KokkosKernelsConfig.cmake fixes for TPLs

[jjwilke]

• Guard add_target with ifs to make sure not added twice
• Misc fixes for the append config line function

[ndellingwood]

@jjwilke what issues did you run into to require this change and in what types of builds?

[jjwilke]

Related to kokkos/kokkos#2587. After fixing this for Kokkos, realized KokkosKernels would have the same problem.

Testing on CUDA builds, I caught the other issue.

[jjwilke]

If you have multiple calls to find_package(KokkosKernels) or find_dependency(KokkosKernels) CMake would load KokkosKernelsConfig.cmake twice and calling add_library twice for the same library.

This was never actually an issue for any KokkosKernels projects, but would have been.

[ndellingwood]

@jjwilke thanks for the explanation, shall I merge into develop or will that break your current work-flow?

[jjwilke]

This should merge into develop ASAP.

[ndellingwood]

@jjwilke merged!