libhio: specify use of external json (#15209)

libhio has been building it's own json.  With this change, libhio will rely on the json installed by spack.

var/spack/repos/builtin/packages/libhio/package.py

@@ -57,4 +57,6 @@ def configure_args(self):
         if '+hdf5' in spec:
             args.append('--with-hdf5={0}'.format(spec['hdf5'].prefix))
 
+        args.append('--with-external-json={0}'.format(spec['json-c'].prefix))
+
         return args