[Qt] Use correct library paths for prefix builds on Mac.

https://bugs.webkit.org/show_bug.cgi?id=120635

Reviewed by Tor Arne Vestbø.

Due to a change of scope of the qmake force_independent configuration
flag, the library paths in QtWebKit builds on the Mac are set to the
QtWebKit build directory even for production builds.

This patch sets the correct library paths for prefixed production builds
while keeping the scope of the force_independent flag for non-production
builds.

* Source/api.pri:
* Source/widgetsapi.pri:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@155034 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Change-Id: I2563e5e7f9582128f32bae459df54186ff88fe87
Task-number: QTBUG-32417
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>