[Qt] Make build-webkit always do safe incremental builds after update-webkit

[Qt] Make build-webkit always do safe incremental builds after update-webkit

When building against Qt5 with GCC we would just run 'make' in the build dir,
and rely on the dependency files output by GCC (-MD) to handle dependency
tracking, but that fails for special-cases like adding a Q_OBJECT macro
to a header.

To guarantee that an incrmental build will work, we have to run 'make qmake',
which we now do on every build-webkit that's followed by a successful run
of update-webkit. The reasoning is that update-webkit can result in such
potential corner-cases being applied, and since we can't know for sure
unless we inspect the diff and account for all the corner cases we assume
the worst and always run 'make qmake'.

After a succesful run of build-webkit we proceed to do just 'make' for any
subsequent runs, since we assume that the developer knows what kind of
changes he/she is doing, and when a 'make qmake' is needed.

Reviewed by Simon Hausmann.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105398 268f45cc-cd09-0410-ab3c-d52691b4dbfc