Custom build variable

I'm trying to automace my build process using batchmode. Part of this process is to set internal version of application.

I tried following approach:

 class BuildScript {
    [UnityEditor.MenuItem("Builder/Build")]
       static void Build() {
           MyConstants.GAME_VERSION = "ver_1.2.3"
           EditorUserBuildSettings.SwitchActiveBuildTarget(BuildTarget.StandaloneOSXIntel);
         string res = BuildPipeline.BuildPlayer(scenes, path, BuildTarget.StandaloneOSXIntel, BuildOptions.None);
       }
 }

where MyConstants just a class with static variable.

But it seems that BuildPipeline executes new versions of code and all changes made in build script are lost.

Without defines with value ala C++ i can't find a way to set variable in code on build. Only way i found so far is to generate GameVersion.cs file with static variable inside every build by hand from build script, but that just too awful.

Is there a better way to accomplish this?