When generating an Xcode project using qmake that's targeting the new
Xcode build system, QMAKE_PRE_LINK does not work properly.
It generates rules that conflict with the default linker rules.
This is a limitation of the new Xcode build system which does not have
any known workaround.
Document the limitation.
Pick-to: 5.15 6.2 6.4 6.5
Fixes: QTBUG-99601
Change-Id: Ie4e6bcb0603ced85f786e9f7f407172e84a00d83
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>