Define an environment variable for use with ifdef and ifndef.
This is simply a macro defined as follows:
(defmacro ifdef-define (x) (cons 'value-triple (cons (cons 'setenv$ (cons x '("1"))) 'nil)))
When this form is loaded, it will set the given environment variable to
Note that the effects of this form are local to the current book; see ifdef-define! for a version that persists across include-books. Also see ifdef-undefine and ifdef-undefine!.