For some reason, fltk-1.1.0-rc5 is not installing the files in the image directory after compiling. The ebuild fails when an operation attempts to modify files which are supposed to be there, but aren't. The image directory isn't even created. I'm not sure if this is a bug with einstall() or with the fltk build process itself.
I have made a couple of fixes to the fltk install. Everything should install correctly now. fltk-1.1.0_rc6 will be up on rsync servers soon. Please test that ebuild and let me know how things go.
It compiles and installs perfectly now, and I used the new USE flags.