For appveyor builds, json_config.h is actually generated in the "include" directory.

appveyor.yml

@@ -30,7 +30,7 @@ after_build:
 - copy json_object_iterator.h include\json-c\*
 - copy json_c_version.h include\json-c\*
 - copy json_inttypes.h include\json-c\*
-- copy json_config.h include\json-c\*
+- copy include\json_config.h include\json-c\*
 - copy json_object_private.h include\json-c\*
 - 7z a json-c.lib.zip lib\json-c.lib include\json-c\*.h