Enable building with clang / libc++ on OS X 10.9 Mavericks

A minimalistic change to enable building with clang / libc++ on OS X
10.9 Mavericks. The configure script now avoids overriding build
settings when using clang/libc++. Added a new build configuration with
settings specific to clang/libc++ on OS X.

Task-number: QTBUG-34652

Change-Id: Iaedaff7c06e9818a5f2ee0d6db93fbe3b2e65e28
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>