- itk-snap 2.4.0 searches for VTK 5.6, but in Gentoo, version 5.10 is the one that usually gets installed.
- FLTK header files and libraries get installed at a different location (/usr/lib/fltk-1 and /usr/include/fltk-1)
Therefore, cmake doesn't automatically find the packages.
The attached patch fixes both issues.
See also Gentoo bug #472786.