[fltk] Fix link to system libs

[JackBoosY]

fltk forgot to export some system libraries in the cmake configuration file on Windows and OSX, fix this.

Fixes #17053.

[JackBoosY]

@MoAlyousef Can you please test this PR?

Thanks.

[SamuelMarks]

@JackBoosY Odd. This worked without any errors, but nothing launched when the compiled executable was run. On the otherhand, my original test-case worked with this slight edit to my CMakeLists.txt:

if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
    find_library(COCOA_LIBRARY Cocoa ONLY)
    target_link_libraries(${PROJECT_NAME} PRIVATE ${COCOA_LIBRARY})
endif()

EDIT: Try this fix of mine on the previous tree not the one after this PR is applied… otherwise it won't work

[MoAlyousef]

I'm afraid I can't test on macos at the moment

[JackBoosY]

@SamuelMarks This modification adapts #16706.

[SamuelMarks]

@JackBoosY / @vicroms : I'm happy this has merged, because it resolved the errors. But it hasn't solved the referenced issue #17053; the GUI window still isn't drawn on my screen.

[JackBoosY]

@SamuelMarks Can you please open a new issue to report this?

Thanks.