multiple fixes and minor additions

* fixed json.j includes for new location in recent library versions
* fixed server string building
* added json-c library to version checking

This changes for the json-c include paths will need to be done other places json-c is used as well.  The symlink from include/json-c to include/json is no longer made by the installer as of the most recent release.  I'll try and put another pull request in for rlm_rest later today.