Add build error for AnyCPU on .NET Framework

tools/nuget/CSFML/CSFML.csproj

@@ -34,6 +34,7 @@
 
   <ItemGroup>
     <None Pack="True" Include="runtimes/**" PackagePath="runtimes" />
+    <None Pack="True" Include="build/**" PackagePath="build" />
   </ItemGroup>
 
 </Project>

tools/nuget/CSFML/build/net/CSFML.props

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+    <PropertyGroup>
+        <CSFMLIgnoreAnyCPU>false</CSFMLIgnoreAnyCPU>
+    </PropertyGroup>
+</Project>

tools/nuget/CSFML/build/net/CSFML.targets

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+    
+    <Target Name="CSFMLAnyCPUError" Condition="$(CSFMLIgnoreAnyCPU) != true and $(Platform) == 'AnyCPU'" BeforeTargets="Build">
+        <Error Text="CSFML doesn't support 'Any CPU' on the .NET Framework. Consider switching to .NET Core. If this is not possible, change your platform to x64 or x86, or set the MSBuild property 'CSFMLIgnoreAnyCPU' to true and manually load the appropriate native library at runtime." />
+    </Target>
+
+</Project>