MOAR printf logging!

`-v:diag` looks reasonable, in that `$(_MonoNativePath)` contained a
value which looked like what I expected.

Yet the build still fails:

    In file included from /Users/runner/work/1/s/src/java-interop/java-interop-gc-bridge-mono.cc:10: (TaskId:621)
    /Users/runner/work/1/s/src/java-interop/java-interop-mono.h:6:10: fatal error: 'mono/metadata/assembly.h' file not found (TaskId:621)
    #include <mono/metadata/assembly.h> (TaskId:621)
             ^~~~~~~~~~~~~~~~~~~~~~~~~~ (TaskId:621)
     (TaskId:621)

I must assume that while the properties have expected values, the
*contents on disk* are…missing?  Or something?

Start logging what's there.

Author: jonpryor

build-tools/automation/templates/core-build.yaml

@@ -19,4 +19,4 @@ steps:
   displayName: Build Solution
   inputs:
     projects: Java.Interop.sln
-    arguments: '-c $(Build.Configuration) -v:diag'
+    arguments: '-c $(Build.Configuration)'

src/java-interop/java-interop.targets

@@ -74,6 +74,16 @@
       <_DMonoLinkFlags>"-DMONO_LINK_FLAGS=$(_MonoLinkFlags)"</_DMonoLinkFlags>
       <_ExtraArgs>$([MSBuild]::Escape('$(_DJdkDirs) $(_DJni_c) $(_DEnableMono) $(_DMonoDirs) $(_DMonoLinkFlags) $(_DEnableOsxArchitectures)'))</_ExtraArgs>
     </PropertyGroup>
+    <Message Text="# jonp: _MonoNativePath: $(_MonoNativePath)" Importance="High" />
+    <PropertyGroup>
+      <_HaveMonoNativePath Condition=" Exists('$(_MonoNativePath)') ">True</_HaveMonoNativePath>
+    </PropertyGroup>
+    <Message Text="# jonp: Does _MonoNativePath exist?: $(_HaveMonoNativePath)" Importance="High" />
+    <ItemGroup>
+      <_MonoIncludeFiles Include="$(_MonoIncludePath)\**\*" />
+    </ItemGroup>
+    <Message Text="# jonp: _MonoIncludeFiles: @(_MonoIncludeFiles, '
+')" Importance="High" />
   </Target>
 
   <Target Name="_BuildJni_c"