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
fixed: compile cjson object into libopm-json

i see no reason to keep the separate library and this circumvents
shared vs static linking issues.

an alternative fix is to fix the opm buildsystems to prioritize config
mode. that would be more involved.
Merge pull request #366 from akva2/fix_cjson

fixed: compile cjson object into libopm-json
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' of ssh://github.com/dr-robertk/opm-parser