+1 on what DiapDealer said about Webkit - the instructions for building Qt for Sigil don't say to do that so not quite sure why you did?
Having pored over the code in detail pretty much every day for the last few months I really can't recommend anyone use either the 0.5.3 codebase or indeed Qt 4.7.4 as the basis for tweaking. You are going to inherit a whole lot of bugs and pain that will distract from your own changes you want to make, stuff that had already been sorted out for 0.6.
Instead just download the official Qt 4.8.2 package built for VS 2010 and install that, no need to build Qt yourself on Windows. Then just follow the rest of the instructions for cmake to get the VS 2010 solution and what I suggested above using the latest Sigil source code. At least that way if you want to reapply your changes against a final Sigil beta/release you will be in a far better position for doing so than against 0.5.3.
Given we are still changing the latest code daily you would in a perfect world be better off waiting for an official release point. The latest checked-in code doesn't have any showstopper bugs we have found in it so far apart from an intermittent crash issue with Replace All (that may be PCRE related).