Use the force-- I mean, the make!

Makes `make json-check` a thing, and further teaches the makefile how to
`make chkjson` (or chkjson.exe, on Windows targets). Moves these out
from doc/contrib to src/chkjson, since they are (well) source files.

The new chkjson still depends on its own .cpp, the stub .h,
json.{h,cpp}, and the Makefile, which consists of the minimum
possible attack vector for Jenkins pull request security.