If you don't actually need to play with Qt, unlike this thread implies:
https://github.com/koreader/koxtoolchain
If you do need to play with Qt, good luck. If you do, AFAICT, for Qt4, you're better off matching Sergey's toolchain, which is probably documented somewhere... For Qt5, Kobo's latest toolchain, which you can suss out from current FW binaries.