Better documentation to compile the hello world example about silicon

[matt-42]

The cmake rules add_iod_executable automatically call the preprocessor but you can still compile it without cmake. You are only missing the first C preprocessor pass:

g++ -E main.cc -I ../install/include/ -std=c++14 -o main2.cc
../install/bin/iodc main2.cc main3.cc
g++ -std=c++14 main3.cc -I ../install/include/ -lmicrohttpd -o hello_world

You can also try the plain c++ version of hello_world

#include <iod/symbol.hh>

iod_define_symbol(hello, _hello);
iod_define_symbol(message, _message);

#include <silicon/api.hh>
#include <silicon/mhd_serve.hh>

using namespace sl;

// Define the API:
auto hello_api = make_api(

  // The hello world procedure.
  s::_hello = [] () { return D(s::_message = "Hello world."); }

);

int main()
{
  // Serve hello_api via microhttpd using the json format:
  sl::mhd_json_serve(hello_api, 12345);
}

[matt-42]

The compilation of hello_world with CMake is documented here:
https://github.com/matt-42/silicon#compilation