[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: svn://svn.chromium.org/blink/trunk@137139 bbb929c8-8fbe-4397-9dbb-9b2b20218538