Unfortunately, that’s yet another problem with the old software under the hood of Elive 3. If you want Sigil, you have to compile it along with its dependencies, which are quite a few:
Elive 3.0 comes with gcc 4.7, which is too old for Sigil, but fortunately enough, there’s a backport of 4.9 in the repository, which you can install with the following command: apt-get install gcc-4.9-backport
Once you do that, you’ll be ready to move onto the other steps to compile Sigil. I’m not sure it’s worth the effort, though. The compilation of the other packages might run into library compatibility issues that you’ll have to deal with one by one, so I would rather install Lubuntu 18.04.1 in a virtual machine and use Sigil there. If you plan using it for productive stuff, it’s actually a good idea to separate your work environment from everything else.
