the current eselect-wxwidgets is eselect-wxwidgets-20180529, which owns the file /usr/share/aclocal/wxwin.m4. x11-libs/wxGTK-3.1.5 tries to overwrite this file, leading to failure. Both earlier versions of wxGTK I have installed require eselect-wxwidgets, so this is preventing upgrade. Reproducible: Always
The Gentoo repository does not contain a wxGTK-3.1 because it's unstable ABI. The ebuild in whatever overlay this comes from needs to be fixed instead, nothing the wxWidgets team can do here.