[driver] Compilation platform is now nullable by default and validated to the host platforms.

src/Core/Compilation.cs

@@ -12,7 +12,7 @@ namespace CppSharp
         /// <summary>
         /// Target platform for code compilation.
         /// </summary>
-        public TargetPlatform Platform;
+        public TargetPlatform? Platform;
 
         /// <summary>
         /// Specifies the VS version.

src/Core/Platform.cs

@@ -77,5 +77,22 @@ namespace CppSharp
                 return platform == PlatformID.Unix || platform == PlatformID.MacOSX;
             }
         }
+
+        public static TargetPlatform Host
+        {
+            get
+            {
+                if (IsWindows)
+                    return TargetPlatform.Windows;
+    
+                if (IsMacOS)
+                    return TargetPlatform.MacOS;
+    
+                if (IsLinux)
+                    return TargetPlatform.Linux;
+    
+                throw new NotImplementedException();
+            }
+        }
     }
 }

src/Generator/Driver.cs

@@ -48,6 +48,9 @@ namespace CppSharp
 
         void ValidateOptions()
         {
+            if (!Options.Compilation.Platform.HasValue)
+                Options.Compilation.Platform = Platform.Host;
+
             foreach (var module in Options.Modules)
             {
                 if (string.IsNullOrWhiteSpace(module.LibraryName))