In XCode 9 beta ios 11 system header files define PLATFORM_TVOS and PLATFORM_IOS - this causes major breakage when compiling for IOS in ue4.
Applications/Xcode-beta.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.0.sdk/usr/include/mach-o/loader.h:1239:9: note: previous definition is here
#define PLATFORM_TVOS 3