% WebKit/WebKitTools/Scripts/build-webkit --debug the build script immediately started complaining about qmake not understanding the "-r" option. This seemed really strange to me as I hadn't thought Safari was a Qt application; it certainly looks much better than any Qt application I've ever seen on the Mac, and I've written 3. It turns out that Safari is not a Qt application but the build script does look for the QTDIR environment variable and will try to build a Qt version if it finds that variable. [Thanks to Mark Rowe on the WebKit team for this information].
So the solution to my problem was to temporarily undefine QTDIR.
% echo $QTDIR
% unsetenv QTDIR
% WebKit/WebKitTools/Scripts/build-webkit --debug
