PBXProject.SetCompileFlagsForFile

Устанавливает флаги компиляции для указанного файла в указанном объекте.

using UnityEditor;
using System.Collections.Generic;
using System.IO;
using UnityEditor.Callbacks;
using UnityEditor.iOS.Xcode;
using System;

public class Sample_SetCompileFlagsForFile  
{
    [PostProcessBuild]
    public static void OnPostprocessBuild(BuildTarget buildTarget, string pathToBuiltProject)
    {

        // Stop processing if build target is not iOS
        if (buildTarget != BuildTarget.iOS)
            return;

        // Initialize PBXProject
        string projectPath = PBXProject.GetPBXProjectPath(pathToBuiltProject);
        PBXProject pbxProject = new PBXProject();
        pbxProject.ReadFromFile(projectPath);

        // Get the target GUID
        string targetGuid = pbxProject.GetUnityFrameworkTargetGuid();

        // Specify the file path you want to get compile flags of (within the Xcode project)
        string filePath = "Plugins/ExampleFile.mm";

        // Retrieve the file GUID using the file path
        string fileGuid = pbxProject.FindFileGuidByProjectPath(filePath);

        // Get the compile flags for the specified file
        string[] compileFlags = pbxProject.GetCompileFlagsForFile(targetGuid, fileGuid).ToArray();
        
        // Check if the file already has a compile flag you want to specify
        string customCompileFlag = "exampleFlag";
        if (Array.IndexOf(compileFlags, customCompileFlag) <  0)
        {
            // Add the compile flag if it doesn't
            pbxProject.SetCompileFlagsForFile(targetGuid, fileGuid, new List<string> (){customCompileFlag});
        }

        // Apply changes to the pbxproj file
        pbxProject.WriteToFile(projectPath);
    }
}