Add support for LTO in xcode builds

indra/cmake/Variables.cmake

@@ -162,6 +162,9 @@ if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
   set(CMAKE_XCODE_ATTRIBUTE_GCC_FAST_MATH YES)
   set(CMAKE_XCODE_ATTRIBUTE_GCC_STRICT_ALIASING NO)
   set(CMAKE_XCODE_ATTRIBUTE_GCC_SYMBOLS_PRIVATE_EXTERN YES)
+  if(USE_LTO)
+    set(CMAKE_XCODE_ATTRIBUTE_LLVM_LTO YES_THIN)
+  endif()
 
   set(CMAKE_XCODE_ATTRIBUTE_GCC_GENERATE_DEBUGGING_SYMBOLS YES)
   set(CMAKE_XCODE_ATTRIBUTE_DEBUG_INFORMATION_FORMAT dwarf-with-dsym)