Enforce strict prototypes.

CMakeLists.txt

@@ -240,6 +240,7 @@ if ("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wextra")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wwrite-strings")
     set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wno-unused-parameter")
+    set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} -Wstrict-prototypes")
 
     add_definitions(-D_GNU_SOURCE)
 elseif ("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC")

json_object.c

@@ -1373,7 +1373,7 @@ static int json_array_equal(struct json_object *jso1, struct json_object *jso2)
 	return 1;
 }
 
-struct json_object *json_object_new_null()
+struct json_object *json_object_new_null(void)
 {
 	return NULL;
 }

json_object.h

@@ -940,7 +940,7 @@ JSON_EXPORT int json_object_set_string_len(json_object *obj, const char *new_val
  * along the lines of the other json_object_new_* functions.
  * It always returns NULL, and it is entirely acceptable to simply use NULL directly.
  */
-JSON_EXPORT struct json_object *json_object_new_null();
+JSON_EXPORT struct json_object *json_object_new_null(void);
 
 /** Check if two json_object's are equal
  *