build: rm products from tree, fix .gitignore

* Remove products of the build from the tree
* Fix typo in the .gitignore lines which refer to them
  - remnant of the pre-submodule era, I guess

All in the name of making cleanup cleaner for use as a submodule.
Fix build failure due to deprecations

json_object_object_get() is deprecated now, and produces warnings.
Treating warnings as errors was causing independent building to break.
Use json_object_object_get_ex() instead - it's been advised since
json-c 0.10 at least.