Logging support: update code

Logging is now controlled by an environment variable (WEBKIT_DEBUG) like
in the other ports.

http://webkitgtk.org/reference/webkitgtk/stable/webkit-environment.html

This is only enabled for builds with NDEBUG unset. You can
add_definitions(-UNDEBUG) in the CMake build files to get such a build.
This will also enable asserts, but leave optimizations on and no debug
information.

NDEBUG is also unset for debug builds, so you can use
-DCMAKE_BUILD_TYPE=Debug in the cmake command-line to get a debug build.
Note this also requires enabling "shared core" mode to get libraries of
a workable size (the cmake script will error out if you don't enable
shared core).
DumpRenderTree: do not deref the gTestRunner.

This was leading to a double free, or an assertion failure before it
happens if assertions are enabled.

Now DumpRenderTree can run several tests in a row, which will allow us
to actually run the testsuite.
Packaging: add xcconfig files and Tools

* xccconfig are part of the XCode project, but the CMake build system will use
them to extract the current WebKit version (as versionned by Apple).
* Tell CPack to use tar.bz2 output format (using package on an archive
that big doesn't work very well), to export the Tools dir and sources
for all ports. There is little value in stripping the package more than
that, and the list of sources isn't always up to date.
StillImageHaiku: fix function prototype

The uperclass added an extra argument to the prototype, and the default
behavior is to recurse infinitely, leading to http://dev.haiku-os.org/ticket/10213

Fix the prototype so the function is called and Web+ doesn't crash
anymore.
Fix NULL dereference.

Apparently, We don't get the context handle when processing downloads.