[webkit-dev] Updated build scripts

[Oliver Hunt]

I just landed a change (r28568) to provide finer granularity when  
enabling and disabling the more advanced SVG features we have in the  
tree (it's no longer an all on or all off setting).  Unfortunately due  
to the significant impact this has on a number of the autogenerated  
files it is necessary to perform a completely clean build.

For Mac and Windows WebKit this will require deleting the WebKitBuild  
directory, unfortunately i am not familiar enough with the qt or gtk  
biulds to know exactly what directory will need to be cleaned :(

Sorry for the inconvenience,
   Oliver