[mtouch] Fix building watch extensions with the watchOS 6.0 SDK.

The watchOS 6.0 SDK renames/moves a few symbols, so some magic is required to
make things work on both watchOS 6.0 and earlier versions.

tools/common/CompilerFlags.cs

@@ -17,6 +17,7 @@ namespace Xamarin.Utils
 		public HashSet<string> LinkWithLibraries; // X, added to Inputs
 		public HashSet<string> ForceLoadLibraries; // -force_load X, added to Inputs
 		public HashSet<string> OtherFlags; // X
+		public List<string> InitialOtherFlags; // same as OtherFlags, only that they're the first argument(s) to clang (because order matters!). This is a list to preserve order (fifo).
 		public HashSet<string> Defines; // -DX
 		public HashSet<string> UnresolvedSymbols; // -u X
 		public HashSet<string> SourceFiles; // X, added to Inputs
@@ -99,6 +100,13 @@ namespace Xamarin.Utils
 			AddOtherFlag ("-stdlib=libc++");
 		}
 
+		public void AddOtherInitialFlag (string flag)
+		{
+			if (InitialOtherFlags == null)
+				InitialOtherFlags = new List<string> ();
+			InitialOtherFlags.Add (flag);
+		}
+
 		public void AddOtherFlag (string flag)
 		{
 			if (OtherFlags == null)
@@ -221,6 +229,16 @@ namespace Xamarin.Utils
 		{
 			Prepare ();
 
+			if (InitialOtherFlags != null) {
+				var idx = 0;
+				foreach (var flag in InitialOtherFlags) {
+					args.Insert (idx, flag);
+					idx += flag.Length;
+					args.Insert (idx, " ");
+					idx++;
+				}
+			}
+
 			ProcessFrameworksForArguments (args);
 
 			if (LinkWithLibraries != null) {

tools/mtouch/Target.cs

@@ -1551,6 +1551,12 @@ namespace Xamarin.Bundler
 				if (App.IsWatchExtension) {
 					mainlib = "libwatchextension.a";
 					linker_flags.AddOtherFlag (" -e _xamarin_watchextension_main");
+					if (App.SdkVersion.Major >= 6) {
+						// watchOS 6.0's WatchKit contains a WKExtensionMain function, and that's the entry point for Xcode-compiled watch extensions.
+						// To make watch extensions work on earlier watchOS versions, there's a libWKExtensionMainLegacy.a library with a
+						// a WKExtensionMain function that does what's needed (Xcode links with this library when deployment target < 6.0).
+						linker_flags.AddOtherInitialFlag ("-lWKExtensionMainLegacy");
+					}
 				} else if (App.IsTVExtension) {
 					mainlib = "libtvextension.a";
 				} else if (App.IsExtension) {

tools/mtouch/mtouch.cs

@@ -672,6 +672,14 @@ namespace Xamarin.Bundler
 					sw.WriteLine ("\txamarin_register_modules = xamarin_register_modules_impl;");
 					sw.WriteLine ("}");
 
+					if (app.Platform == ApplePlatform.WatchOS && app.SdkVersion.Major >= 6) {
+						sw.WriteLine ();
+						sw.WriteLine ("extern \"C\" { int WKExtensionMain (int argc, char* argv[]); }");
+						sw.WriteLine ("int main (int argc, char *argv[])");
+						sw.WriteLine ("{");
+						sw.WriteLine ("\treturn WKExtensionMain (argc, argv);");
+						sw.WriteLine ("}");
+					}
 				}
 				WriteIfDifferent (main_source, sb.ToString (), true);
 			} catch (MonoTouchException) {