cmake: add json support

[junghans]

[junghans]

@markcmiller86 I got the cmake logic done, but json-c-0.10 is like 10+ year old and the more recent json-c-0.18 has a different includedir (json/ -> json-c/) and a slightly different library interface.

I would recommend to use json-0.18 or so and just use the distribution package instead of build it yourself.

[markcmiller86]

I would recommend to use json-0.18 or so and just use the distribution package instead of build it yourself.

Well, that is potentially problematic because what Silo is using is a enhanced json-c library with some features not available in the standard json release. I will give it some thought though.

[junghans]

I see, the best might be to hide that cmake option for the time being and figure this out after the release.

[markcmiller86]

#510 replaces this