In XCode 5, 6 and 7, you can still set the variable.
Navigate to Build Settings
A. Select your project on the left side bar under Project Navigator.
B. Select the “Build Settings” tab.
Find and set
A. Select ‘All’ in the upper left of the main screen.
B. Search for
MOMC_NO_INVERSE_RELATIONSHIP_WARNINGSby pasting the key into the search bar.
C. Change value to YES.
Build again and the warnings are gone.
Thanks @RajTandel for the comment. If you still see the warnings, restart Xcode and the warnings should go away.