Ah... I'm still on 11.10... and without LD_LIBRARY_PATH updated with my custom-built Qt-4.8 libs (4.7.4 is default), Sigil v6.x just flat-out wouldn't launch without it after a successful build/install.
Other than that, we're basically doing the same thing: you're setting the session PATH to pick up Qt5's qmake and I'm using update-alternatives to automatically manipulate a generic qmake symlink in /usr/bin so it points to the particular version I want to build against.
I guess I should bite the bullet and update someday, but my hardware is getting pretty old.
