bug#23795: 24.5; rx eval on variable gives symbol error

[Noam Postavsky]

rx is a macro so it doesn't see runtime bindings, using
let-when-compile (new in Emacs 25) works:

(let-when-compile ((page-name "the page"))
  (rx "[[" (eval page-name) "]]"))

Not sure if the plain let case can/should be fixed.