For google and posterity:
If you are trying to compile something in xcode, and your project has files that are named the same as system files, like math.h, and you have problems with xcode including your math.h, even when you use #include <math.h> instead of “math.h”, then you have come to the right place.
You’d think you just need to uncheck ALWAYS_SEARCH_USER_PATHS, but no, because magical .hmap files will apparently still map math.h to your math.h no matter what you do.
No, the fix is to add a a user defined setting to your target:
USE_HEADERMAP = NO
Thank you Apple, for this nice bit of xcode magic that has wasted so many hours of my life. Twice. And for not including as a standard checkbox feature in project settings, so one might stumble upon it and fix the problem without having to pull out ones hair for 3 hours over it. Twice.
Bitter? Who me?