We should not copying the headers in the make step of the derived sources since the generated headers that we need is not exists that time. We should do that in the main make step. The change that we need to achieve this is to move the section of the header copies from WebCore.pri - what is included from DerivedSources.pro - to WebCore.pro.
Created attachment 63056 [details] proposed patch
If you copy the headers in the main build step, then you loose the dependency tracking for files that _use_ these header files in turn. I think this step should remain in DerivedSources.pro, but instead of copying I suggest to _create_ header files that consist of a simple #include line that includes the real header file. This way the step in DerivedSources.pro does not depend on the other file generators and as a bonus you'd get _real_ dependency tracking from the WebKit2 sources to the _actual_ Webcore headers.
Comment on attachment 63056 [details] proposed patch I'm going to say r- because of the other suggested approach. If you still want to use this approach, then I think at least the addExtraCompiler duplication should be replaced with a direct use of QMAKE_EXTRA_COMPILERS, as I don't think any of these copy steps use any of the features that addExtraCompiler() adds to the regular qmake extra compiler handler.