Includepaths are sometimes modified by non-Qt contributors so keeping
them in files inside Sources makes it more likely that they are updated
along with project files for the other ports.

Using pri files instead of prf files for this also has the benefit that
the include() from the main target file can be parsed and followed by
Qt Creator -- something that does not work with load().

Dependency from a target to a library through the WEBKIT variable are
handled through forwarding-files in Tools/qmake/mkspecs/modules, which
set the source root of the module and include the right pri file.

Ideally we'd use the variant of include() that takes an optional
namespace to read the variables into, or the fromfile() function,
but both of these add an overhead of about 40% on the total qmake
runtime, due to making a deep copy of all the variables in the
project or re-reading all the prf files from scratch.

Reviewed by Simon Hausmann.
Reviewed by Ossy.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@104285 268f45cc-cd09-0410-ab3c-d52691b4dbfc