[vtkusers] build all modules except the ones from system

[Nico Schlömer]

Hi everyone,

I noticed that the combination
```
-DVTK_BUILD_ALL_MODULES=ON
-DVTK_USE_SYSTEM_JSONCPP=ON
```
makes VTK build ThirdParty/jsoncpp. I don't want that: Just take it from
system.

How can I configure VTK such that all modules are built, _except_ the ones
that I take from system?

Cheers,
Nico
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