ENH: Add option on using system library

[alanliu02]

Tested on Windows10, MSVC16, VS2019.
There are some issues with DCMTK. When compling DCMTK related plugins, it is required to put compiled libraries in some project folder to be able to continue compling.

[alanliu02]

Solving issues like #911

[lassoan]

We have been using CTK with VTK, ITK, etc. built outside of CTK for many years in 3D Slicer, as you can specify these variables as command-line arguments. So, adding the options is not necessary. However, it does not hurt to add these options. Maybe you could mark them as "advanced" to prevent cluttering the CMake GUI.

[alanliu02]

We have been using CTK with VTK, ITK, etc. built outside of CTK for many years in 3D Slicer, as you can specify these variables as command-line arguments. So, adding the options is not necessary. However, it does not hurt to add these options. Maybe you could mark them as "advanced" to prevent cluttering the CMake GUI.

@lassoan Thanks for the hint, I think it's necessery to let advanced user to know that this option is available.
Since the Internet connection can sometimes be wrong, one will need this option to prevent building CTK with other thirdparty libraries online, instead they can chose to compile thirdparty libraries first with local source and let CMake find exist libraries.
I now marked these options as advanced and I think it fits your mind. Thanks for the review!

[lassoan]

Thank you, it looks good to me.

[lassoan]

If you squash the commits into one (and make sure the commit comment is valid) then it should be good to go.

[alanliu02]

I think it's ready to merge @lassoan @pieper

[pieper]

LGTM