libjson-rpc-cpp: enable parallel building

pkgs/development/libraries/libjson-rpc-cpp/default.nix

···
              -DCMAKE_BUILD_TYPE=Release
   '';
 
-  installPhase = '' 
+  installPhase = ''
     mkdir -p $out
 
     function fixRunPath {
···
 
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ cmake jsoncpp argtable curl libmicrohttpd doxygen catch ];
+
+  enableParallelBuilding = true;
 
   meta = with stdenv.lib; {
     description = "C++ framework for json-rpc (json remote procedure call)";