Add MIRROR define and version (#437)

* Add MIRROR define and version

* Update PreprocessorDefine.cs

Assets/Mirror/Editor/PreprocessorDefine.cs

@@ -0,0 +1,32 @@
+using System.Collections.Generic;
+using System.Linq;
+using UnityEditor;
+using UnityEngine;
+
+namespace Mirror
+{
+    static class PreprocessorDefine
+    {
+        /// <summary>
+        /// Symbols that will be added to the editor
+        /// </summary>
+        public static readonly string[] Symbols = new string[] {
+            "MIRROR",
+            "MIRROR_1726_OR_NEWER"
+        };
+
+        /// <summary>
+        /// Add define symbols as soon as Unity gets done compiling.
+        /// </summary>
+        [InitializeOnLoadMethod]
+        static void AddDefineSymbols()
+        {
+            string definesString = PlayerSettings.GetScriptingDefineSymbolsForGroup(EditorUserBuildSettings.selectedBuildTargetGroup);
+            List<string> allDefines = definesString.Split(';').ToList();
+            allDefines.AddRange(Symbols.Except(allDefines));
+            PlayerSettings.SetScriptingDefineSymbolsForGroup(
+                EditorUserBuildSettings.selectedBuildTargetGroup,
+                string.Join(";", allDefines.ToArray()));
+        }
+    }
+}

Assets/Mirror/Editor/PreprocessorDefine.cs.meta

@@ -0,0 +1,11 @@
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