This RJSONIO package uses the libjson C library from libjson.sourceforge.net. A version of that C++ 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. To do this, you 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 OS X) environment variable.