[GTK] No IPC messages are sent when building WebKitGTK+ with VERSION_SCRIPT
https://bugs.webkit.org/show_bug.cgi?id=135760

Reviewed by Philippe Normand.

The problem is that the threading initialization is failing
because there are two copies of WTF, one in libjavascriptcoregtk
and the other in libwebkit2gtk. When WebKit2 is initialized in the
UI process, JSC::initializeThreading() is called first and then
WTF::initializeMainThread(). The former is calling
ThreadIdentifierData::initializeOnce() initializing the
ThreadIdentifierData::m_key symbol in libjavascriptcoregtk, while
the latter is using the ThreadIdentifierData API from libwebkit2gtk
that hasn't been initialized.

* CMakeLists.txt: Do not add WTF to the list of WebKit2 libraries,
WebKit2 already depends on JavaScriptCore that already links to WTF.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@172385 268f45cc-cd09-0410-ab3c-d52691b4dbfc