Define OR_GREATER for .NET Standard versions when targeting .NET Core/Framework

NETSTANDARD_OR_GREATER.md

So, I had this code that I wanted to work properly:

namespace AbbLab.Compatibility
{
    public static class Shims
    {
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static T[] Empty<T>()
#if NETSTANDARD1_3_OR_GREATER
            => System.Array.Empty<T>();
#else
            => EmptyArray<T>.Instance;

        private static class EmptyArray<T>
        {
            internal static readonly T[] Instance = new T[0];
        }
#endif
    }
}

But when targeting .NET 5.0, I noticed that NETSTANDARD1_3_OR_GREATER was not defined for some reason, even though .NET 5.0 implements .NET Standard 2.1. I spent several days trying to figure out what's wrong with my .NET SDK's installation. Turns out, everything's working as intended - .NET Core was never supposed to have OR_GREATER symbols of .NET Standard. So I spent a day digging in MSBuild's docs and files, and invented a work-around:

<Project>
  <!-- This is the Directory.Build.props file, in the root of your solution -->

  <PropertyGroup Condition="'$(TargetFramework)' != '' and ('$(TargetFrameworkIdentifier)' == '' or '$(TargetFrameworkVersion)' == '')">
    <TargetFrameworkIdentifier>$([MSBuild]::GetTargetFrameworkIdentifier('$(TargetFramework)'))</TargetFrameworkIdentifier>
    <TargetFrameworkVersion>v$([MSBuild]::GetTargetFrameworkVersion('$(TargetFramework)', 2))</TargetFrameworkVersion>
  </PropertyGroup>

  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp'">
    <!-- .NET Core 1.0+ implements .NET Standard 1.0 through 1.6 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '1.0'))">$(DefineConstants);NETSTANDARD1_0_OR_GREATER;NETSTANDARD1_1_OR_GREATER;NETSTANDARD1_2_OR_GREATER;NETSTANDARD1_3_OR_GREATER;NETSTANDARD1_4_OR_GREATER;NETSTANDARD1_5_OR_GREATER;NETSTANDARD1_6_OR_GREATER</DefineConstants>
    <!-- .NET Core 2.0+ implements .NET Standard 2.0 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '2.0'))">$(DefineConstants);NETSTANDARD2_0_OR_GREATER</DefineConstants>
    <!-- .NET Core 3.0+ implements .NET Standard 2.1 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '3.0'))">$(DefineConstants);NETSTANDARD2_1_OR_GREATER</DefineConstants>
  </PropertyGroup>

  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
    <!-- .NET Framework 4.5+ implements .NET Standard 1.0 through 1.1 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '4.5'))">$(DefineConstants);NETSTANDARD1_0_OR_GREATER;NETSTANDARD1_1_OR_GREATER</DefineConstants>
    <!-- .NET Framework 4.5.1+ implements .NET Standard 1.2 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '4.5.1'))">$(DefineConstants);NETSTANDARD1_2_OR_GREATER</DefineConstants>
    <!-- .NET Framework 4.6+ implements .NET Standard 1.3 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '4.6'))">$(DefineConstants);NETSTANDARD1_3_OR_GREATER</DefineConstants>
    <!-- .NET Framework 4.6.1+ implements .NET Standard 1.4 through 2.0 -->
    <DefineConstants Condition="$([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '4.6.1'))">$(DefineConstants);NETSTANDARD1_4_OR_GREATER;NETSTANDARD1_5_OR_GREATER;NETSTANDARD1_6_OR_GREATER;NETSTANDARD2_0_OR_GREATER</DefineConstants>
  </PropertyGroup>
</Project>

With this, NETSTANDARDX_Y_OR_GREATER will be defined for versions of Core and Framework that implement those standards.