[Qt] Add a configure step to the Qt build system

[Qt] Add a configure step to the Qt build system

This allows building the Qt port using just 'qmake WebKit.pro'. Using
the build-webkit script is still supported, and will add slightly more
logic to the build, such as detecting the need for clean or incremental
builds.

Internally, the build system now uses a new variable, WEBKIT_CONFIG, for
all things related to configuring the build, such as use/have/enable flags,
and these are translated to defines in default_post. Project files should
no longer check contains(DEFINES,...) to detect features, but use the new
enable?(), use?(), and have?() functions. The no_webkit1 and no_webkit2
options have been translated into WEBKIT_CONFIG options as well, and can
be checked using build?().

Reviewed by Simon Hausmann.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@128174 268f45cc-cd09-0410-ab3c-d52691b4dbfc