[Qt] Builds for Mac are tied to the OS system instead of the SDK version.

https://bugs.webkit.org/show_bug.cgi?id=104529

Reviewed by Simon Hausmann.

Using work by Eike Ziller <eike.ziller@digia.com>.

The build system now first tries to assign the correct system libraries
for the SDK being used for Mac builds. If this does not yield any
result, it falls back to using the Mac OS version to determine the
correct library.

No new tests (not applicable).

* WebCore.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@137139 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Change-Id: I2ece3075a797f07c6774c508252bd735c625314b
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Merge branch 'stable' of git://qt.gitorious.org/qt/qtwebkit into st

Conflicts:
	Source/WebCore/WebCore.pri