Build with XCode 10's "New Build System" is broken

Qt Bug reference

QTBUG-71035

• Builds fail from now as the Legacy Build System is removed
• Fix version in commercial qt: 5.15.6

Message of commit to cherry pick

Now that inputs (81152194) and outputs (3f0858ed) are explicitly set for
the preprocess stage we can enable the new build system.

Using the legacy build system will produce a build error in Xcode 13,
but the build will succeed:

  error: The Legacy Build System will be removed in a future release.
  You can configure the selected build system and this deprecation
  message in File > Project Settings.

Fixes: QTBUG-71035
Pick-to: 6.1 5.15 5.12
Change-Id: I108d2103872255d10de2ff5161eef892065da1c4
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>


(cherry picked from commit 3d7bdf0d61699cb7fdfcef7fe7b546e3167723b2)