This RJSONIO package uses the libjson C++ library from
libjson.sourceforge.net (there are other projects called libjson).  A
version of that code is included in this package and can be used.
Alternatively, one can use a different version of libjson, e.g. a more
recent version (but that site has not been updated since 2012, version
7.6.1, a corrected version of which is included here).  To do this, you
can build and install that version of libjson with

     make SHARED=1 install

The key thing is to make this a shared library so we can link against
it as position independent code (PIC).

By default, this will be installed in /usr/local.
You can control this with

     make SHARED=1 install prefix="/my/directory"


The configuration script will attempt to find libjson on your system,
looking in /usr/local. If this is not the location you installed
libjson to, you can specify the prefix via the --with-prefix=/my/directory,
e.g. 
   R CMD INSTALL --configure-args="--with-prefix=/my/directory" RJSONIO

If you want to force using the version of libjson contained in this package,
you can use

   R CMD INSTALL --configure-args="--with-local-libjson=yes" RJSONIO


If you do use a version of libjson installed into a non-standard
place, you will probably need to modify your LD_LIBRARY_PATH (or
DYLD_LIBRARY_PATH on macOS) environment variable.