Fix the native JSON support code

Fix the native JSON support code

* src/Makefile.in (base_obj): Add the missing json.o.  Without
this, we get link error.
* src/json.c (json_serialize): Don't use too sophisticated C99
features, as they confuse make-docfile.  Initialize all the
members explicitly.

(cherry picked from commit 000f919b3c7779609dc43773fdc49aca9b50d76f)