fixed: properly encapsulate cjson
changed: cjson handling

- probe only for system lib (i.e. only probe in prefix paths)
old code ended up not installing the json lib due to finding a copy
in the build folder if a reconfiguration was performed.

- build internal copy static and bundle in libopm-json.so
no reason to install this as a shared library. if it is updated
originating from opm, the entire parser library is updated anyways.
added: cmake config mode and pkgconfig
Merge pull request #365 from akva2/fix_cjson

changed: cjson handling