bug#13509: wrong "definition in expression context" in R6RS mode

[Marco Maggi <marco.maggi-ipsu@poste.it>]

On my i686-pc-linux-gnu running:

    $ guile --version
    guile (GNU Guile) 2.1.0.1329-d7cb7

built from the head of the "master" branch, this program:

    #!r6rs
    (import (rnrs))
    (define (alpha)
      (define-syntax define-special
        (syntax-rules ()
          ((_ ?who ?val)
           (define ?who ?val))))
      (define-special beta #t)
      #f)
    (alpha)

should succeed, but instead it fails with:

    ;;; compiling /home/marco/var/tmp/proof.sps
    ;;; WARNING: compilation of /home/marco/var/tmp/proof.sps failed:
    ;;; ERROR: Syntax error:
    ;;; /home/marco/var/tmp/proof.sps:12:2:   definition  in
    ;;; expression context, where definitions a re not allowed, in form (define beta #t)
    ice-9/boot-9.scm:718:27: In procedure map:
    ice-9/boot-9.scm:718:27: Syntax error:
    /home/marco/var/tmp/proof.sps:12:2: definition in expression context, where definitions are not allowed, in form (define beta #t)
-- 
Marco Maggi