Declaring a variable safe

[Eli Zaretskii]

The way to specify that a variable is safe for certain categories of
values is by giving it a `safe-local-variable' property.  For example:

  (put 'foobar 'safe-local-variable 'booleanp)

But what is the recommended way of doing this with a variable that is
defined in the C sources?  One possibility is to put the above form
somewhere in a Lisp file that is preloaded, like simple.el.  Is there
a better way?

TIA