Every time CMake is re-run, Xcode shows a popup asking if
user wants to manage schemes automatically or manually.
Building Blender wiki page recommends managing schemes automatically.
This change sets the default behavior to "automatically" and generates
the .xcscheme files while CMake is running, instead of hogging Xcode
later on. With tests enabled, the number of schemes is 203.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D8820
5eb5978043