macx-ios-clang: only provide default LaunchScreen.xib if using Xcode 6

If using an older version of Xcode, Xcode will sometimes complain
that LaunchScreen.xib uses auto layout while the project at
the same time has deployment target set to 5.0 (where auto layout
is not supported).
This is a bug in Xcode really, since LaunchScreen.xib will only be
used when running on iOS 7 (otherwise a LaunchImage will be used).
This has been fixed in Xcode 6.

This patch adds a check for this early on.

Change-Id: Ie612c25b413add23e15fc3cb4f9e30bb5292369d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>