Are you sure you haven't (manually) relocated your Qt5 installation after building Sigil? Even with the LD_LIBRARY_PATH env var updated to the new Qt5 location, I've seen segfaults under those conditions. It seems the path to the Qt5 platform plugins is hard coded in the various libraries (never mind that the platform plugin doesn't show up as a dependency when using ldd). You could try adding the line
to your launch script (before the call to launch Sigil) to see if you get some more useful output to work with.