[msbuild] Fix metal targets for Xcode8 (xamarin#1838)

Fixes bug #52982: [iOS]Metal samples fail to build with Xcode8.3
(https://bugzilla.xamarin.com/show_bug.cgi?id=52982)

Basically with Xcode8 Apple stopped using an intermediary step to generate the
default.metallib. This was what our `_ForgeMetal` target was doing, generate a `default.metal-ar`
file which was used as input for `_TemperMetal` and then generate the default.metallib.

Instead with Xcode8 you can just give Shaders.air directly to the metallib tool.

The fix in this commit is made in such a way that it still supports Xcode7.

if !Xcode8 then don't change anything.
if Xcode8+ then have `_ForgedMetal` output equal `@(_SmeltedMetal)` (basically skip the _ForgeMetal target).

msbuild/Xamarin.Mac.Tasks.Core/Tasks/DetectSdkLocationsTaskBase.cs

@@ -49,6 +49,11 @@ public string SdkVersion {
 			get; set;
 		}
 
+		[Output]
+		public bool IsXcode8 {
+			get; set;
+		}
+
 		#endregion Outputs
 
 		public override bool Execute ()
@@ -60,6 +65,8 @@ public override bool Execute ()
 			EnsureXamarinSdkRoot ();
 			EnsureSdkPath ();
 
+			IsXcode8 = AppleSdkSettings.XcodeVersion.Major >= 8;
+
 			return !Log.HasLoggedErrors;
 		}
 

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.targets

@@ -352,13 +352,21 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 	<Target Name="_ForgeMetal" Condition="'$(_CanOutputAppBundle)' == 'true' And '@(_SmeltedMetal)' != ''" DependsOnTargets="_SmeltMetal"
 		Inputs="@(_SmeltedMetal)" Outputs="$(IntermediateOutputPath)metal\default.metal-ar">
 		<ArTool
-			Condition="'$(IsMacEnabled)' == 'true'"
+			Condition="'$(IsMacEnabled)' == 'true' and '$(_IsXcode8)' == 'false'"
 			SessionId="$(BuildSessionId)"
 			Items="@(_SmeltedMetal)"
 			Archive="$(IntermediateOutputPath)metal\default.metal-ar">
 		</ArTool>
 
-		<CreateItem Include="$(IntermediateOutputPath)metal\default.metal-ar">
+		<!-- If !Xcode8 -->
+		<CreateItem Include="$(IntermediateOutputPath)metal\default.metal-ar"
+					Condition="'$(_IsXcode8)' == 'false'">
+			<Output TaskParameter="Include" ItemName="_ForgedMetal" />
+		</CreateItem>
+
+		<!-- Else -->
+		<CreateItem Include="@(_SmeltedMetal)"
+					Condition="'$(_IsXcode8)' == 'true'">
 			<Output TaskParameter="Include" ItemName="_ForgedMetal" />
 		</CreateItem>
 	</Target>
@@ -447,6 +455,7 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 			<Output TaskParameter="SdkBinPath" PropertyName="_SdkBinPath" />
 			<Output TaskParameter="SdkDevPath" PropertyName="_SdkDevPath" />
 			<Output TaskParameter="SdkUsrPath" PropertyName="_SdkUsrPath" />
+			<Output TaskParameter="IsXcode8" PropertyName="_IsXcode8" />
 		</DetectSdkLocations>
 	</Target>
 

msbuild/Xamarin.iOS.Tasks.Core/Tasks/DetectSdkLocationsTaskBase.cs

@@ -69,6 +69,11 @@ public bool SdkIsSimulator {
 			get; set;
 		}
 
+		[Output]
+		public bool IsXcode8 {
+			get; set;
+		}
+
 #endregion Outputs
 
 		public PlatformFramework Framework {
@@ -92,6 +97,8 @@ public override bool Execute ()
 
 			SdkIsSimulator = (architectures & (TargetArchitecture.i386 | TargetArchitecture.x86_64)) != 0;
 
+			IsXcode8 = AppleSdkSettings.XcodeVersion.Major >= 8;
+
 			EnsureAppleSdkRoot ();
 			EnsureXamarinSdkRoot ();
 			switch (Framework) {

msbuild/Xamarin.iOS.Tasks.Core/Xamarin.iOS.Common.targets

@@ -496,12 +496,20 @@ Copyright (C) 2013-2016 Xamarin. All rights reserved.
 		Inputs="@(_SmeltedMetal)" Outputs="$(DeviceSpeficicIntermediateOutputPath)metal\default.metal-ar">
 		<ArTool
 			SessionId="$(BuildSessionId)"
-			Condition="'$(IsMacEnabled)' == 'true'"
+			Condition="'$(IsMacEnabled)' == 'true' and '$(_IsXcode8)' == 'false'"
 			Items="@(_SmeltedMetal)"
 			Archive="$(DeviceSpecificIntermediateOutputPath)metal\default.metal-ar">
 		</ArTool>
 
-		<CreateItem Include="$(DeviceSpecificIntermediateOutputPath)metal\default.metal-ar">
+		<!-- If !Xcode8 -->
+		<CreateItem Include="$(DeviceSpecificIntermediateOutputPath)metal\default.metal-ar"
+					Condition="'$(_IsXcode8)' == 'false'">
+			<Output TaskParameter="Include" ItemName="_ForgedMetal" />
+		</CreateItem>
+
+		<!-- Else -->
+		<CreateItem Include="@(_SmeltedMetal)"
+					Condition="'$(_IsXcode8)' == 'true'">
 			<Output TaskParameter="Include" ItemName="_ForgedMetal" />
 		</CreateItem>
 	</Target>
@@ -559,6 +567,7 @@ Copyright (C) 2013-2016 Xamarin. All rights reserved.
 			<Output TaskParameter="SdkUsrPath" PropertyName="_SdkUsrPath" />
 			<Output TaskParameter="SdkPlatform" PropertyName="_SdkPlatform" />
 			<Output TaskParameter="SdkIsSimulator" PropertyName="_SdkIsSimulator" />
+			<Output TaskParameter="IsXcode8" PropertyName="_IsXcode8" />
 		</DetectSdkLocations>
 	</Target>