Well, I tried it myself and for me it didn't work w/o specifying both include dirs. I only tested under Linux, but with both make and ninja (both failed). Everything worked fine with the include dirs. I also removed the GUI_TYPE variable and set both GUI attributes in the add_executable() statement. The resultant (shorter) file is then (with hello.cxx as source file):

cmake_minimum_required(VERSION 3.14)
project(app)

include(FetchContent)

set(FLTK_BUILD_TEST OFF CACHE BOOL " " FORCE)
if(UNIX AND NOT APPLE) 
    set(OPTION_USE_PANGO ON CACHE BOOL " " FORCE)
endif()

FetchContent_Declare(FLTK
    GIT_REPOSITORY https://github.com/fltk/fltk
)
FetchContent_MakeAvailable(FLTK)

add_executable(main WIN32 MACOSX_BUNDLE hello.cxx)
target_include_directories(main PRIVATE ${fltk_BINARY_DIR} ${fltk_SOURCE_DIR})
target_link_libraries(main PRIVATE fltk) # if needed: fltk_gl fltk_images fltk_png fltk_jpeg fltk_z 

if(APPLE)
    target_link_libraries(main PRIVATE "-framework Cocoa") # need for Darwin
endif()

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