Unity editor script which compiles itself on command, including OS X when PATH doesn't include the target's folder.

CompileTest.cs

using UnityEngine;
using UnityEditor;
using System.Diagnostics;
using Debug = UnityEngine.Debug;
using System.IO;


public class CompileTest : ScriptableObject
{
	[MenuItem ("Assets/Run compiler test")]
	static void Test ()
	{
		CompileTest instance = CreateInstance<CompileTest> ();
		string path = Application.dataPath + AssetDatabase.GetAssetPath (MonoScript.FromScriptableObject (instance)).Substring ("Assets".Length);
		DestroyImmediate (instance);

		string arguments = "-target:library ";
		arguments += path;
		arguments += " -r:\"" + Path.Combine (UnityAssemblyPath, "UnityEngine.dll") + "\"";
		arguments += " -r:\"" + Path.Combine (UnityAssemblyPath, "UnityEditor.dll") + "\"";

		Process process = null;
		try
		{
			process = Process.Start (CreateCompilerStartInfo (arguments));
			process.WaitForExit ();
		}
		catch (System.Exception e)
		{
			string errorMessage = e.GetType ().Name + ": ";

			if (e is System.ComponentModel.Win32Exception)
			{
				errorMessage += "Win32 error code " + ((System.ComponentModel.Win32Exception)e).NativeErrorCode + ": ";
			}

			errorMessage += e.Message;

			Debug.LogError ("Process exception (" + e.GetType ().Name + "): " + errorMessage);

			return;
		}

		if (!process.StandardError.EndOfStream)
		{
			string error = process.StandardError.ReadToEnd ();

			Debug.LogError ("Process error: " + error);

			return;
		}

		Debug.Log ("Success");

		AssetDatabase.Refresh ();
	}



	public static bool OsIsX
	// Assuming we are never executing on non-OS X Unix
	{
		get
		{
			int p = (int)System.Environment.OSVersion.Platform;
			return (p == 4 || p == 6 || p == 128);
		}
	}


	public static string CompilerPath
	{
		get
		{
			return EditorApplication.applicationContentsPath +
				(OsIsX ? "/Frameworks/Mono/bin/gmcs" : "/Mono/bin/gmcs.bat");
		}
	}


	public static string UnityAssemblyPath
	{
		get
		{
			return (Application.platform == RuntimePlatform.WindowsEditor) ?
				Path.Combine (
					EditorApplication.applicationContentsPath,
					"Managed"
				)
			:
				Path.Combine (
					Path.Combine (
						EditorApplication.applicationContentsPath,
						"Frameworks"
					),
					"Managed"
				);
		}
	}


	static string ProperPath (string path)
	{
		return path.Replace (Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar);
	}


	public static ProcessStartInfo CreateCompilerStartInfo (string arguments)
	{
		ProcessStartInfo startInfo;

		if (OsIsX)
		{
			startInfo = new ProcessStartInfo ()
			{
				WorkingDirectory = Path.GetDirectoryName (CompilerPath),
				FileName = Path.GetFileName (CompilerPath)
			};

			string currentPath = System.Environment.GetEnvironmentVariable("PATH");
			if(currentPath == null || !currentPath.Contains(Path.GetDirectoryName (CompilerPath))) {
				System.Environment.SetEnvironmentVariable("PATH", (currentPath != null ? currentPath+":" : "")+Path.GetDirectoryName (CompilerPath));
			}
		}
		else
		{
			startInfo = new ProcessStartInfo ()
			{
				FileName = ProperPath (CompilerPath)
			};
		}

		startInfo.Arguments = arguments;
		startInfo.UseShellExecute = false;
		startInfo.RedirectStandardError = true;
		startInfo.CreateNoWindow = true;

		return startInfo;
	}
}