Hi, this is a minor issue but pretty visible.  In latest fltk from
github, every run of fltk-config prints "grep:
/nix/store/c20mk84ykvyfzixcxr0yg41sbrxsrfx4-fltk-1.4-c0f5080cb056919e898ac572e72fd62568dc307a/bin/FL/fl_config.h:
No such file or directory" which is quite frequent if build files are using it.
I tracked this down to 6546814a23,

    Oh, yeah that's mine.. was to get the new cairo example in examples/cairo-draw-x.cxx
    to build, as we don't currently have a way to detect cairo any other way. (the old
    technique was flawed too, which checked for the presence of the .lib)

    Just pushed b275ff071 which should solve that -- can you try that and follow up?

    Yes, the right solution is to tweak 'configure' to set a variable and have that be
    expanded into fltk-config.

    I'm afraid I don't know 'configure' at all, so I'm not the one that could implement
    that change -- I think Albrecht mentioned it was something he was going to adapt.