In Xcode 4 you can set compiler flags for a single file from the target’s Compile Sources build phase.

1. Select the project file from the project navigator to open the project editor.
2. Select the target from the left side of the project editor.
3. Click the Build Phases button at the top of the editor.
4. Click the disclosure triangle next to the Compile Sources build phase.

Select the file where you want to set the compiler flags. Double-click the Compiler Flags column to open a pop-up editor, which is where you enter the flags.

Tags: xcode 4