Add NETSTANDARD define to the FSharp compilation, attempt to get multitargeting working

src/FSharp/AssemblyInfo.fs

@@ -48,7 +48,7 @@ open System.Runtime.InteropServices
 [<assembly: AssemblyFileVersion("3.20.0.0")>]
 [<assembly: AssemblyInformationalVersion("3.20.0")>]
 
-#if PORTABLE
+#if PORTABLE || NETSTANDARD
 #else
 [<assembly: ComVisible(false)>]
 [<assembly: Guid("048BC4EB-CE2B-4040-9967-4784F5405B0F")>]

src/FSharp/BigIntegerExtensions.fs

@@ -1,6 +1,6 @@
 namespace System.Numerics
 
-#if PORTABLE
+#if PORTABLE || NETSTANDARD
 #if NOSYSNUMERICS
 #else
 

src/FSharp/FSharp-2017.fsproj

@@ -1,8 +1,6 @@
-<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
-    <!--<TargetFrameworks>net45;netstandard1.6</TargetFrameworks>-->
-    <!--<TargetFramework>netstandard1.6</TargetFramework>-->
-    <TargetFramework>net45</TargetFramework>
+    <TargetFrameworks>net45;netstandard1.6</TargetFrameworks>
     <DisableImplicitFSharpCoreReference>true</DisableImplicitFSharpCoreReference>
     <AssemblyName>MathNet.Numerics.FSharp</AssemblyName>
     <RootNamespace>MathNet.Numerics</RootNamespace>
@@ -11,10 +9,28 @@
     <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(TargetFramework)'=='netstandard1.6'">
+      <DefineConstants>NETSTANDARD;NOSYSNUMERICS</DefineConstants>
+  </PropertyGroup>
+
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+      <DebugSymbols>true</DebugSymbols>
+      <DebugType>full</DebugType>
+      <Optimize>false</Optimize>
+      <Tailcalls>false</Tailcalls>
+      <OutputPath>bin\Debug\</OutputPath>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+      <DebugType>pdbonly</DebugType>
+      <Optimize>true</Optimize>
+      <Tailcalls>true</Tailcalls>
+      <OutputPath>bin\Release\</OutputPath>
+  </PropertyGroup>
+
   <Import Project="$(FSharpTargetsPath)" Condition="Exists('$(FSharpTargetsPath)')" />
   <ItemGroup>
-    <Reference Include="mscorlib" />
-    <PackageReference Include="FSharp.Core" Version="4.2.*" Private="True" />
+    <PackageReference Include="FSharp.NET.Sdk" Version="1.0.*" PrivateAssets="All" />
+    <PackageReference Include="FSharp.Core" Version="4.2.*" PrivateAssets="All" />
     <ProjectReference Include="..\Numerics\Numerics.csproj" />
   </ItemGroup>
   <ItemGroup>

src/FSharp/Random.fs

@@ -49,7 +49,7 @@ module Random =
     let inline system () = SystemRandomSource() :> System.Random
     let inline systemSeed (seed:int) = SystemRandomSource(seed) :> System.Random
 
-#if PORTABLE
+#if PORTABLE || NETSTANDARD
 #else
     /// Creates a default .Net cryptographic system pRNG
     let inline crypto () = new CryptoRandomSource() :> System.Random