2.16.0 and 2.16.1 change with #{ #} syntax

[Jay Anderson]

In 2.16.0 creating a book inside #{ #} returned the book. In 2.16.1 and
2.16.2 this is no longer always the case. Here's an example which fails:

============
\version "2.16.2"

makeBook =
#(define-void-function (parser location) ()
  (let ((the-book
    #{
      \book
      {
        \include "header.ily"
        \score
        {
          \new Staff
          {
            c'1
          }
        }
      }
    #}))
    (display the-book) ;;; in 2.16.2 #<undefined>
    (newline)
    (print-book-with-defaults parser the-book)))

\makeBook
============

header.ily just contains:
============
\header
{
}
============

If you take the \include out or put the header inline it compiles, but that
defeats the purpose.

I just tried 2.17.10 and the behavior is fixed there.

-----Jay