lang/racket lang/racket-textual: review dependencies

* lang/racket: add missing dependency on GTK3+ needed by the
  `drracket' graphical IDE. Pointed out by @netbsdnoob on UnitedBSD.
  See https://docs.racket-lang.org/gui/libs.html
* Explicitly enable curses and iconv support. These are set upstream
  to be enabled if corresponding headers are found, which means enabled
  on NetBSD, but not necessarily elsewhere. As a consequence, buildlink
  include libiconv.
* Use pkgsrc devel/libffi instead of the bundled library to avoid
  duplication.
* Again, bump revision.

(vins)