* Add module for cJSON
* Add module for json-c

as we now support cJSON and json-c we need to wrap the functions we use.
also allows drop in replacements for older cJSON versions lacking
certain functions.

use the new WinPR JSON wrapper API

Currently, the `arm_handle_bad_request` function returns `FALSE` when the
`cJSON_ParseWithLength` function fails to parse the message, but only when
the `cJSON_GetErrorPtr` returns a valid pointer. It would be better to
return regardless of the `cJSON_GetErrorPtr` return value.

Although, the `pkg_check_modules` function is used when finding the
json-c library, the results are never used. Let's add the `HINTS` params
for the `find_path` and `find_library` functions.

This is in preparation for the subsequent commit adding test case for
the prefs functionality to avoid building the code twice.

The WINPR_JSON_FOUND macro is never defined currently. Consequently,
the SDL prefs always use the fallback implementation. Let's use the
WITH_WINPR_JSON instead to really parse the sdl-freerdp.json file.

* pkg-config sets CJSON_FOUND, check for that instead of the paths, which
  might be unset or set to <var>-NOTFOUND
* move detection to own file. find_package targets are meant to be used
  in same directory or below, but we define the WinPR target one above,
  so include it in parent