Never mind. Totally my fault.
I forgot about the change that made Qt-4.8 mandatory. I've compiled
Qt-4.8.2 and I can build
against 4.8.2, but as far as my system-wide settings are concerned... Qt-4.7.4 still rules the roost, so to speak.
I'll just have to remember that Sigil's "make install" doesn't do me a lot of good and go back to using the bash script to launch Sigil and tell it where my custom Qt-4.8 libs live.
Sorry for the confusion -- and now on to playing!!