Silence redundant Intel compiler warning
Silence redundant Intel compiler warning

Turn off warning about unused template parameter, when the
parameter is used for SFINAE tests inside the template
parameter list in the json library.

halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor