recommend JSON::XS instead of conditionally requiring it

re https://rt.cpan.org/Public/Bug/Display.html?id=84876

This patch eliminates the whole "does it have JSON::XS and have C-compiler?" in Makefile.PL and instead just recommends JSON::XS.

I kept the `pp_only` logic there, and am not sure if this should also be taken into account and even remove it from `recommends` if pp-only is in effect.