Explicitly specify FX version

Author: hazzik

doc/NHibernate.shfbproj

@@ -1,4 +1,5 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="14.0">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
   <PropertyGroup>
     <!-- The configuration and platform will be used to determine which
          assemblies to include from solution and project documentation
@@ -14,6 +15,7 @@
     <RootNamespace>Documentation</RootNamespace>
     <Name>Documentation</Name>
     <!-- SHFB properties -->
+    <FrameworkVersion>.NET Framework 4.6.1</FrameworkVersion>
     <OutputPath></OutputPath>
     <HtmlHelpName>NHibernateAPI</HtmlHelpName>
     <HelpFileFormat>HtmlHelp1</HelpFileFormat>
@@ -35,6 +37,10 @@
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
   </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="EWSoftware.SHFB.NETFramework" Version="4.8.0.2" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.Common.targets" Condition="'$(MSBuildRestoreSessionId)' != ''" />
   <!-- Import the SHFB build targets -->
-  <Import Project="$(SHFBROOT)\SandcastleHelpFileBuilder.targets" />
+  <Import Project="$(SHFBROOT)\SandcastleHelpFileBuilder.targets" Condition="'$(MSBuildRestoreSessionId)' == ''" />
 </Project>