[cli] Improve platform detection so the current platform is set by default.

src/CLI/Generator.cs

@@ -25,8 +25,24 @@ namespace CppSharp
             this.options = options;
         }
 
+        static TargetPlatform GetCurrentPlatform()
+        {
+            if (Platform.IsWindows)
+                return TargetPlatform.Windows;
+
+            if (Platform.IsMacOS)
+                return TargetPlatform.MacOS;
+
+            if (Platform.IsLinux)
+                return TargetPlatform.Linux;
+
+            throw new System.NotImplementedException("Unknown host platform");
+        }
+
         public bool ValidateOptions(List<string> messages)
         {
+            if (!options.Platform.HasValue)
+                options.Platform = GetCurrentPlatform();
             if (Platform.IsWindows && options.Platform != TargetPlatform.Windows)
             {
                 messages.Add("Cannot create bindings for a platform other that Windows from a Windows running machine");

src/CLI/Options.cs

@@ -29,7 +29,7 @@ namespace CppSharp
 
         public string InputLibraryName { get; set; }
         
-        public TargetPlatform Platform { get; set; } = TargetPlatform.Windows;
+        public TargetPlatform? Platform { get; set; }
 
         public TargetArchitecture Architecture { get; set; } = TargetArchitecture.x86;