Use new Directory.Build.targets MSBuild 15 system.

Directory.Build.targets

<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <!-- Import any parent target files. -->
  <Import Condition="Exists('$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory).., $(MSBuildThisFile)))\$(MSBuildThisFile)')" Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory).., $(MSBuildThisFile)))\$(MSBuildThisFile)"/>

  <!-- Include your logic here for the current level. -->
</Project>

proj.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  ...
  <PropertyGroup>
    <_ImportDirTarget>$([MSBuild]::GetDirectoryNameOfFileAbove('$(MSBuildThisFileDirectory)', 'Directory.Build.targets'))\Directory.Build.targets</_ImportDirTarget>
  </PropertyGroup>
  <Import Condition="'$(MSBuildToolsVersion)' &lt; '15' and Exists('$(_ImportDirTarget)')" Project="$(_ImportDirTarget)"/>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  ...
 </Project>

proj.csproj modifications are needed if you are unsure if building on msbuild < 15.