Tigervnc-1.10.1 Build error

I'm on March's Slackware64-Current, which comes thinly equipped with vnc stuff and of course does not automagically sort package dependencies. Code:/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx: In constructor 'Viewport::Viewport(int, int, const rfb::PixelFormat&, CConn*)':
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx:129:19: error: 'fl_display' was not declared in this scope; did you mean 'display'?
129 | xkb = XkbGetMap(fl_display, 0, XkbUseCoreKbd);
| ^~~~~~~~~~
| display
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx: In member function 'void Viewport::setLEDState(unsigned int)':
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx:429:26: error: 'fl_display' was not declared in this scope; did you mean 'display'?
429 | ret = XkbLockModifiers(fl_display, XkbUseCoreKbd, affect, values);
| ^~~~~~~~~~
| display
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx: In member function 'void Viewport::pushLEDState()':
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx:481:24: error: 'fl_display' was not declared in this scope; did you mean 'display'?
481 | status = XkbGetState(fl_display, XkbUseCoreKbd, &xkbState);
| ^~~~~~~~~~
| display
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx: In member function 'unsigned int Viewport::getModifierMask(unsigned int)':
/home/dec/download/tigervnc-1.10.1/vncviewer/Viewport.cxx:682:19: error: 'fl_display' was not declared in this scope; did you mean 'display'?
682 | xkb = XkbGetMap(fl_display, XkbAllComponentsMask, XkbUseCoreKbd);
| ^~~~~~~~~~
| display
make[2]: *** [vncviewer/CMakeFiles/vncviewer.dir/build.make:184: vncviewer/CMakeFiles/vncviewer.dir/Viewport.cxx.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:909: vncviewer/CMakeFiles/vncviewer.dir/all] Error 2
make: *** [Makefile:147: all] Error 2Tigervnc needs fltk. It's the one dependency I lacked. I made a package and shoved in the latest stable one (1.3.5) and got a fltk related error. So I grabbed the latest git (Version 1.4.0-wannabe) and get the error above, which also appears to be fltk related. fl_display is used all over the vncviewer/ subdir in Tigervnc.

I'm posting here rather than on the tigervnc site in case anyone knows it. I'll have to resurrect my google account to post on the tigervnc forums, and have them in my face for a while.

EDIT: fl_display always seems to have the keyboard in the line e.g. vncviewer.Viewport.cxx line 481:
status = XkbGetState(fl_display, XkbUseCoreKbd, &xkbState);