[Qt] Make use of .qmake.cache for caching features

[Qt] Make use of .qmake.cache for caching features

Instead of loading() features from the files that need them (and re-running
a bunch of checks), we now run feature detection as part of configure.pro,
and have build-webkit write the computed feature-defines and CONFIG to
.qmake.cache, which is then loaded by qmake _before_ even defaults_pre
when building WebKit.pro.

At some point we'll be able to selectivly prevent running of config tests
in configure.pro, which means we don't need a separate code-path for
the build-webkit --help case.

We should also move the code in build-webkit that now uses .webkit.config
to detect clean builds, to use .qmake.cache, since we now store the same
thing there.

Original patch by Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

https://bugs.webkit.org/show_bug.cgi?id=90461

Reviewed by Tor Arne Vestbø.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121766 268f45cc-cd09-0410-ab3c-d52691b4dbfc