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The qmake build system

Basic structure

The qmake build is split into three different file types:

    * .pro files: These files represent top level targets that can be
    built individually, or a collection of sub-targets used for
    organizing the project.

    * .pri files: These files are included from top level targets,
    and represent 'implementation details' of how the target is built.

    * .prf files: These files take care of generic build rules
    that applies to all targets, or specific 'features' that can
    be loaded on demand.

The first two file types are spread throughout the source tree, while
the .prf files are located in 'Tools/qmake/mkspecs/features' and will
get loaded by qmake based on setting the QMAKEPATH environment variable.

Root project file

The root project file 'WebKit.pro' is handy both for loading WebKit
in Qt Creator, and for building QtWebKit. Normally you will build
using build-webkit, but you can also run qmake directly on the root
project file. Just make sure to set QMAKEPATH first, so that the
custom mkspecs are picked up.

Feature files

Feature files (.prf files) are used in the following ways:

    1. Every time qmake parses a project file, it will first load
    a special feature file called 'defaults_pre.prf', then parse
    the project file, and then load another special feature file
    called 'defaults_post.prf'. We use these special files to set
    default options that all project files use, expose a few handy
    functions, and to post-process the build config based on what
    the project file did.

    2. Dependencies on other targets (libraries) are declared by
    using CONFIG+=othertarget. This will add the correct include
    paths and linker options to use the library.

    3. Optional features can be enabled by passing CONFIG+=foo on
    the command line when running qmake on the root project file,
    (or by passing --make-args="CONFIG+=foo" to build-webkit). For
    example 'CONFIG+=valgrind'.

Derived sources

Some targets (JavaScriptCore, WebCore, etc) rely on generated files,
(aka. derived sources). These must be generated before the real target
is built. This is achieved by splitting the target up into two sub-
projects, one for the derived sources and one for the real target,
and telling qmake to build them in order using CONFIG += ordered.

The WEBKIT variable

The custom qmake variable 'WEBKIT' is used for signaling that a
target depends in some way on other subproject of the WebKit
project. For now this is limited to the set of intermediate
libraries: wtf, javascriptcore, webcore, and webkit2.

Adding a dependency results in additional include paths being
available, and potentially linking to the library. This is
decided by the build system based on conditions such as what
kind of target is being built and the general build config.