How do I make this simple addition code less verbose?
You may want to take a look at Pythy. 

What you want seems a lot like:

PYTHY(add, x, y) (
    return x + y;
)


Make sure to read this part of the article, as pointed out by Konrad:


  It appears that we are derefencing a null pointer. Remember in C++
  when dereferencing a null pointer, undefined behavior occurs when
  there is an rvalue-to-lvalue conversion. However, since a
  non-capturing lambda closure is almost always implemented as an object
  with no members, undefined behavior never occurs, since it won't
  access any of its members. Its highly unlikely that a non-capturing
  lambda closure could be implemented another way since it must be
  convertible to a function pointer. But perhaps not?