[java-interop] Resuscitate Mono GC Bridge

Insert description here.

`src/java-interop` support for building against MonoVM was broken
at some point, and is arguably still *wrong* because even now it's
using the legacy Mono header files, *not* the current MonoVM headers.

TODO: figure out how to fix that.

*For now*, fix `_CreateMonoInfoProps` so that
`bin/Build$(Configuration)/MonoInfo.props` is created, which in turn
allows `src/java-interop` ***on macOS and Linux*** to use the
mono headers and link against libmonosgen-2.0.dylib/etc.:

	% nm src/java-interop/obj/Debug-net8.0/libjava-interop.dylib | grep _mono
	                 …
	                 U _mono_class_from_mono_type

Currently unknown: how do we *use* `libjava-interop.dylib`?

The 'obvious' way is to build a project with `-p:UseMonoRuntime=true`.
That fails under .NET 9:

	error NU1102: Unable to find package Microsoft.NETCore.App.Runtime.Mono.osx-x64 with version (= 9.0.2)

Try to re-add support for building with .NET 8, by overriding
`$(DotNetTargetFramework)`, which *does* build with `-p:UseMonoRuntime=true`:

```
dotnet build -c Release -t:Prepare -p:DotNetTargetFramework=net8.0 *.sln
dotnet build -c Release -p:DotNetTargetFramework=net8.0 *.sln

dotnet publish --self-contained -p:UseMonoRuntime=true -p:DotNetTargetFramework=net8.0 -p:UseAppHost=true -p:ErrorOnDuplicatePublishOutputFiles=false -r osx-x64 samples/Hello-Java.Base/Hello-Java.Base.csproj

cp samples/Hello-Java.Base/bin/Release/Hello-Java.Base.jar samples/Hello-Java.Base/bin/Release/osx-x64/publish

samples/Hello-Java.Base/bin/Release/osx-x64/publish/Hello-Java.Base
```

Update `JreRuntime.cs` to check for the existence of `Mono.RuntimeStructs`,
as `Mono.Runtime` no longer exists.

I don't yet know if this uses the GC bridge, though.

Author: jonpryor

build-tools/Java.Interop.BootstrapTasks/Java.Interop.BootstrapTasks.targets

@@ -1,12 +1,12 @@
 <Project>
 
   <Target Name="_CreateMonoInfoProps"
-      Condition=" ($([MSBuild]::IsOSPlatform ('linux')) Or $([MSBuild]::IsOSPlatform ('osx'))) And '$(_MonoPath)' != '' "
+      Condition=" ($([MSBuild]::IsOSPlatform ('linux')) Or $([MSBuild]::IsOSPlatform ('osx'))) "
       AfterTargets="AfterBuild"
       DependsOnTargets="_GetLinuxMonoPaths;_GetMacOSMonoPaths"
       Inputs="$(MSBuildThisFileFullPath);$(MSBuildThisFileDirectory)Java.Interop.BootstrapTasks.csproj"
       Outputs="$(_OutputPath)MonoInfo.props">
-    <ItemGroup>
+    <ItemGroup Condition=" Exists($(_MonoPath)) ">
       <_MonoInfoLine Include="&lt;Project&gt;" />
       <_MonoInfoLine Include="  &lt;Choose&gt;" />
       <_MonoInfoLine Include="    &lt;When Condition=&quot; %27%24(MonoFrameworkPath)%27 == %27%27 &quot;&gt;" />
@@ -22,11 +22,15 @@
       <_MonoInfoLine Include="&lt;/Project&gt;" />
     </ItemGroup>
     <WriteLinesToFile
+        Condition=" Exists($(_MonoPath)) "
         File="$(_OutputPath)MonoInfo.props"
         Lines="@(_MonoInfoLine)"
         Overwrite="True"
     />
-    <Touch Files="$(_OutputPath)MonoInfo.props" />
+    <Touch
+        Condition=" Exists($(_MonoPath)) "
+        Files="$(_OutputPath)MonoInfo.props"
+    />
   </Target>
 
   <Target Name="_CreateMonoMk"
@@ -84,6 +88,7 @@
       <_MonoFrameworkPath>$(_MonoLibPath)libmonosgen-2.0.1.dylib</_MonoFrameworkPath>
       <_MonoIncludePath>$(_MonoBase)Headers/mono-2.0</_MonoIncludePath>
       <_MonoLibs>-L "$(_MonoLibPath)" -lmonosgen-2.0</_MonoLibs>
+      <_MonoPath>$(_MonoBase)Commands/mono</_MonoPath>
     </PropertyGroup>
   </Target>
 

src/Java.Interop/Java.Interop/JniRuntime.cs

@@ -445,8 +445,9 @@ public virtual void OnEnterMarshalMethod ()
 		public virtual void OnUserUnhandledException (ref JniTransition transition, Exception e)
 		{
 			transition.SetPendingException (e);
-
+#if NET9_0_OR_GREATER
 			Debugger.BreakForUserUnhandledException (e);
+#endif  // NET9_0_OR_GREATER
 		}
 
 		public virtual void RaisePendingException (Exception pendingException)

src/Java.Runtime.Environment/Java.Interop/JreRuntime.cs

@@ -91,8 +91,9 @@ static unsafe JreRuntimeOptions CreateJreVM (JreRuntimeOptions builder)
 			builder.TypeManager     ??= new JreTypeManager (builder.typeMappings);
 #endif  // NET
 
-			bool onMono = Type.GetType ("Mono.Runtime", throwOnError: false) != null;
+			bool onMono = Type.GetType ("Mono.RuntimeStructs", throwOnError: false) != null;
 			if (onMono) {
+				Console.WriteLine ($"MonoVM support enabled");
 				builder.ValueManager            = builder.ValueManager              ?? new MonoRuntimeValueManager ();
 				builder.ObjectReferenceManager  = builder.ObjectReferenceManager    ?? new MonoRuntimeObjectReferenceManager ();
 			}

src/java-interop/java-interop.targets

@@ -47,7 +47,7 @@
   </Target>
 
   <Target Name="_GetCmakeOptions">
-    <PropertyGroup Condition=" '$(TargetFramework)' == 'net472' And '@(MonoIncludePath->Count())' != '0' ">
+    <PropertyGroup Condition=" '@(MonoIncludePath->Count())' != '0' ">
       <_MonoDirs>"-DMONO_INCLUDE_LIST=@(MonoIncludePath, ';')"</_MonoDirs>
       <_MonoLib>"-DMONO_LINK_FLAGS=$(MonoLibs)"</_MonoLib>
       <_EnableMono>-DENABLE_MONO_INTEGRATION=ON</_EnableMono>