I use update-alternatives to manage multiple qmake versions. Once you have the update-alternatives package installed, it's fairly trivial to add an alternate version of qmake (the one you custom built). It then handles the symlinking necessary when choosing which different versions of qmake (or any program for that matter) you want to use.
All in all, I've never needed more than the /usr/bin/qmake symlink pointed to qmake in the directory where I installed my custom Qt build in order to build Sigil against a version of Qt that wasn't my system default. Update-alternatives just made it a bit easier to manage the symlinks. Of course after it's built, you'll need to launch Sigil from a script that adds your custom Qt/lib directory to the LD_LIBRARY_PATH environment variable.
You've reminded me that I need to be working on getting Qt5 compiled so I'll be able to build any new versions of Sigil down the road.
Last edited by DiapDealer; 01-22-2013 at 06:09 PM.