Variable: cpp-unknown-writable
cpp-unknown-writable is a customizable variable defined in cpp.el.gz.
Value
t
Documentation
Non-nil means you are allowed to modify the unknown conditionals.
Source Code
;; Defined in /usr/src/emacs/lisp/progmodes/cpp.el.gz
(defcustom cpp-unknown-writable t
"Non-nil means you are allowed to modify the unknown conditionals."
:type 'boolean)