Fix Mono 5.0 run under Unity 5.5+

Author: veblush

extra/UniversalCompiler/Compilers/Mono50Compiler.cs

@@ -1,5 +1,6 @@
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 
 internal class Mono50Compiler : Compiler
 {
@@ -11,17 +12,25 @@ protected override Process CreateCompilerProcess(Platform platform, string monoP
 		var systemCoreDllPath = Path.Combine(monoProfileDir, "System.Core.dll");
 
 		string processArguments;
-		if (platform == Platform.Windows)
+		if (platform == Platform.Windows && GetSdkValue(responseFile) == "2.0")
 		{
-			processArguments = $"-sdk:2 -debug+ -langversion:Future -r:\"{systemCoreDllPath}\" {responseFile}";
+			// -sdk:2.0 requires System.Core.dll. but -sdk:unity doesn't.
+			processArguments = $"-r:\"{systemCoreDllPath}\" {responseFile}";
 		}
 		else
 		{
-			processArguments = $"-sdk:2 -debug+ -langversion:Future {responseFile}";
+			processArguments = responseFile;
 		}
 
 		var process = new Process();
 		process.StartInfo = CreateOSDependentStartInfo(platform, ProcessRuntime.CLR40, compilerPath, processArguments, unityEditorDataDir);
 		return process;
 	}
+
+	private string GetSdkValue(string responseFile)
+	{
+		var lines = File.ReadAllLines(responseFile.Substring(1));
+		var sdkArg = lines.FirstOrDefault(line => line.StartsWith("-sdk:"));
+		return (sdkArg != null) ? sdkArg.Substring(5) : "";
+	}
 }