Create Scripting Define Symbol from script for the selected Build Target in Unity.

Define.cs

using System;
using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
public class Define
{


	const string define = "LIBRARY_IS_AVAILABLE";


	static Define()
	{ AddDefineIfNeeded(); }

	static void AddDefineIfNeeded()
	{
		// Get selected target defines.
		BuildTargetGroup buildTargetGroup = EditorUserBuildSettings.selectedBuildTargetGroup;
		string defines = PlayerSettings.GetScriptingDefineSymbolsForGroup(buildTargetGroup);

		// Only if not defined already.
		if (defines.Contains(define))
		{
			Debug.LogWarning("Selected build target ("+EditorUserBuildSettings.activeBuildTarget.ToString()+") already contains <b>"+define+"</b> <i>Scripting Define Symbol</i>.");				
			return; 
		}

		// Append.
		PlayerSettings.SetScriptingDefineSymbolsForGroup(buildTargetGroup, (defines + ";" + define));
		Debug.LogWarning("<b>"+define+"</b> added to <i>Scripting Define Symbols</i> for selected build target ("+EditorUserBuildSettings.activeBuildTarget.ToString()+").");
	}
}