[msbuild] Define __MACOS__ for Xamarin.Mac. Fixes #36258. (#2048)

https://bugzilla.xamarin.com/show_bug.cgi?id=36258

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.AppExtension.CSharp.targets

@@ -21,7 +21,7 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 		<!-- Version/fx properties -->
 		<TargetFrameworkVersion Condition="'$(TargetFrameworkIdentifier)' == '' And '$(TargetFrameworkVersion)' ==''">v4.5</TargetFrameworkVersion>
 
-		<DefineConstants>__UNIFIED__;$(DefineConstants)</DefineConstants>
+		<DefineConstants>__UNIFIED__;__MACOS__;$(DefineConstants)</DefineConstants>
 	</PropertyGroup>
 
 	<Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.CSharp.targets

@@ -22,7 +22,7 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 		<!-- Version/fx properties -->
 		<TargetFrameworkVersion Condition="'$(TargetFrameworkIdentifier)' == '' And '$(TargetFrameworkVersion)' == ''">v4.5</TargetFrameworkVersion>
 
-		<DefineConstants>__UNIFIED__;$(DefineConstants)</DefineConstants>
+		<DefineConstants>__UNIFIED__;__MACOS__;$(DefineConstants)</DefineConstants>
 	</PropertyGroup>
 
 	<Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.ObjCBinding.CSharp.props

@@ -35,7 +35,7 @@ Copyright (C) 2014 Xamarin Inc. All rights reserved.
 		<BTouchToolExe Condition="!Exists('$(BTouchToolPath)/bgen')">bmac-mobile-mono</BTouchToolExe>
 		<AllowUnsafeBlocks>True</AllowUnsafeBlocks>
 
-		<DefineConstants>__UNIFIED__;$(DefineConstants)</DefineConstants>
+		<DefineConstants>__UNIFIED__;__MACOS__;$(DefineConstants)</DefineConstants>
 	</PropertyGroup>
 
 	<Import Project="$(MSBuildThisFileDirectory)$(MSBuildThisFileName).After.props"