Trying to reduce more legacy WebKit2 branding, I first wanted to rename WEBKIT2_COMPILATION to WEBKIT_COMPILATION. But that's almost redundant with BUILDING_WEBKIT, which is used in more places. I don't think we need both. We don't need a separate preprocessor definition to distinguish between building Source/WebKit vs. other components of WebKit because Source/WebKit does not have any export macros, and it would be extremely confusing to try to maintain such a distinction anyway. Defining BUILDING_WEBKIT everywhere simplifies things.

Currently it's actually defined separately for each port, only when building Source/WebKit, but this is confusing and annoying as tests often need to include private headers, so it's easier if we move it to a global location.