Add conditions for implicit framework references for .NET 3.5 and below

Fixes #532

src/Tasks/Microsoft.NET.Build.Tasks/build/Microsoft.NET.Sdk.BeforeCommon.targets

@@ -77,18 +77,18 @@ Copyright (c) .NET Foundation. All rights reserved.
   </PropertyGroup>
 
   <ItemGroup Condition=" '$(DisableImplicitFrameworkReferences)' != 'true' and '$(TargetFrameworkIdentifier)' == '.NETFramework'">
+    <!-- When doing greater than/less than comparisons between strings, MSBuild will try to parse the strings as Version objects and compare them as
+         such if the parse succeeds. -->
     <Reference Include="System"/>
-    <Reference Include="System.Core"/>
+    <Reference Include="System.Core" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '3.5' "/>
     <Reference Include="System.Data"/>
     <Reference Include="System.Drawing"/>
-    <!-- When doing greater than/less than comparisons between strings, MSBuild will try to parse the strings as Version objects and compare them as
-         such if the parse succeeds. -->
     <Reference Include="System.IO.Compression.FileSystem" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '4.5' "/>
     <Reference Include="System.IO.Compression" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '4.5' "/>
     <Reference Include="System.Net.Http" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '4.5' "/>
     <Reference Include="System.Numerics" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '4.0' "/>
-    <Reference Include="System.Runtime.Serialization"/>
-    <Reference Include="System.Xml.Linq"/>
+    <Reference Include="System.Runtime.Serialization" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '3.5' "/>
+    <Reference Include="System.Xml.Linq" Condition=" '$(_TargetFrameworkVersionWithoutV)' >= '3.5' "/>
     <Reference Include="System.Xml"/>
   </ItemGroup>