CMake: make "optional" dependencies 'PUBLIC' (#1173)

On FreeBSD some of the required header files (and libraries) are not
in standard (known) locations, hence they wouldn't be found under
certain circumstances. Making all dependencies 'PUBLIC' adds the
needed include directories and link libraries to all builds.

For details see GitHub Issue #1173 with title
  "FLUID compilation error on FreeBSD when FLTK_BACKEND_X11=ON"