Please update for FLTK 1.4

Describe the bug

Hi,

I tried to compile TigerVNC with FLTK 1.4 and it fails.

To Reproduce Steps to reproduce the behavior:

1. Create a "build" directory in the source tree
2. cd build
3. cmake -G "Unix Makefiles" ..

Expected behavior Successful build

Screenshots

$ cmake -G "Unix Makefiles" ..
[...]
-- Found FLTK: /usr/lib/x86_64-linux-gnu/libfltk_images.so;/usr/lib/x86_64-linux-gnu/libfltk.so
-- Performing Test OK_FLTK_VERSION
-- Performing Test OK_FLTK_VERSION - Failed
CMake Error at CMakeLists.txt:317 (message):
  Incompatible version of FLTK

Client (please complete the following information):

• OS: Debian sid with the libfltk1.4-dev package