birth: Support for cond-expand else

Birth is pretty much at a feature freeze at this point, but `cond-expand'
really was difficult to work with in certain circumstances when it didn't
even support `else'.

* build-aux/bootstrap/birth.scm (fnmap)[cond-expand]: Always expand `else'
    while dropping all others.

build-aux/bootstrap/birth.scm

@@ -378,9 +378,16 @@
       (("js:console")
        (string-append "console.log(" (map sexp->es args) ")"))
 
-      ;; always expands into nothing; this is just to facilitate its use
+      ;; only expands `else' clauses; this is just to facilitate its use
       ;; moving forward
-      (("cond-expand") "")
+      (("cond-expand")
+       (join "" (map (lambda (clause)
+                       (let ((feature (token-value (car clause)))
+                             (body    (cdr clause)))
+                         (if (string=? feature "else")
+                             (body->es body #f)
+                             "")))
+                     args)))
 
       ;; yes, there are more important things to do until we get to the
       ;; point where it's worth implementing proper tail calls