--- Csound5.17.6/InOut/CMakeLists.txt
+++ Csound5.17.6/InOut/CMakeLists.txt
@@ -148,6 +148,7 @@
 if(USE_FLTK)
     set(widgets_SRCS FL_graph.cpp winFLTK.c widgets.cpp)
     make_plugin(widgets "${widgets_SRCS}" "${FLTK_LIBRARIES}")
+    include_directories(${FLTK_INCLUDE_DIR})
 endif()
 
 check_deps(BUILD_VIRTUAL_KEYBOARD FLTK_FOUND)