Xcode: Dynamically choose release/debug libs based on current configuration

Non-framework builds would automatically link to whatever Qt library
matched the config at the time of running qmake, eg hard-coded to
libQtCore_debug, while Xcode itself allowed the user to switch between
release and debug configurations.

We now append an Xcode settings variable to the library path, which gets
resolved at build time depending on the current config in Xcode.

Change-Id: I12873e38a28d9595ef3fd0ae0ad849e6744833a9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Move the Apple Clang specific features outside of the C++11 section

Those are not related to C++11, so move them to the Clang detection
block.

Change-Id: I80b298e1df5f74a865d743625895e0f01cb5fd6b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>