Accommodate Doxygen 1.8.16, as found in experimental (#943426).

* documentation/make_pdf: Loosen caption de-hyperlinking to
  accommodate (but not require) interposed mboxes, as produced by Doxygen
  1.8.16 (currently in experimental).
* documentation/src/fltk-book.tex.in: Explicitly \usepackage{ifthen} for
  the sake of several constructs in doxygen.sty, which has just a
  commented-out \RequirePackage call for it as of 1.8.16.  (Together with
  the above, closes: #943426.)