[vtkusers] build all modules except the ones from system

[Ben Boeckel]

On Tue, Apr 11, 2017 at 18:25:06 +0000, Nico Schlömer wrote:
> 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?

Third party packages should still be taken from the system. The module
is shown as "enabled", but that should just mean that it goes and
forwards system information from the system to the modules which depend
on it. If you try and build the "vtkjsoncpp" target with this setup, the
target shouldn't actually exist.

--Ben