Modified 
 in Source/
   JSONChildren.h  - use namespace std;   - see BDR's note.
   libjson.cpp - change ../../libjson.h  to "libjson/libjson.h"

   JSONStats.h  - change "../../JSONOptions.h" to "JSONOptions.h". This may not be necessary after changes to configure.in and copying files to src/

Then copy to the R package source:

cd src/libjson
cp -f ~/Downloads/libjson/_internal/Source/[A-Za-z]* Source
cp -f ~/Downloads/libjson/_internal/Source/JSONDefs/* Source/JSONDefs

configure copies src/libjson/JSONOptions.h to src/ now.