Best way to support obsolete variables

Question

Emacs 25.1 obsoletes a variable isearch-word and recommends to use the new isearch-regexp-function instead. Because Emacs 24 does not have the new variable, I can't just replace it like:

(if isearch-regexp-function

What's the best way to support both Emacs 24 and Emacs 25?

I don't want to just suppress the warning with with-no-warnings, which is already answered here: "How to avoid obsolete warning when defining a compatibility alias?"

Answer 1

Use boundp to test for the variable's existence:

(boundp SYMBOL)

Return t if SYMBOL's value is not void. Note that if lexical-binding is in effect, this refers to the global value outside of any lexical scope.

So:

(if (boundp 'isearch-regexp-function)
    (do-the-new-thing)
  (do-the-old-thing))