fix begin-for-syntax under splicing-let

This fixes an immediate problem, but the macro expander should have
complained about an unbound `maybe` at phase 2. (A new implementation
of the macro expander detected the unbound `maybe`.)

pkgs/racket-test-core/tests/racket/syntax.rktl

@@ -1634,6 +1634,15 @@
          (define-syntax outer-x (make-rename-transformer #'x)))
         outer-x))
 
+(test 10 'splicing+begin-for-syntax
+      (eval
+       '(begin
+         (splicing-let ([x 10])
+                       (begin-for-syntax
+                         (define id #'x)))
+         (define-syntax (m stx) id)
+         (m))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Check keyword & optionals for define-syntax 
 ;; and define-syntax-for-values:

racket/collects/racket/splicing.rkt

@@ -171,6 +171,11 @@
          [_ body]))]))
 
 (begin-for-syntax
+  (define-for-syntax ((maybe unintro) form)
+    (if (syntax-property form 'definition-intended-as-local)
+        form
+        (unintro form)))
+
   (define-syntax (splicing-let-body/et stx)
     (syntax-case stx ()
       [(_ marked-id markless-id body)