Remove quotes from VB define constants

It looks like MSBuild has a bug on *nix where they aggressively
change directory separators from '\' to '/', even when the '\'
is being used as an escape character in a define constant in VB.
This change works around the bug by removing all quotes from
define constants after Microsoft.VisualBasic.CurrentVersion adds
them in our build. This should be OK since none of our constants
should require quoting.

build/Targets/VSL.Imports.targets

@@ -55,6 +55,10 @@
   <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" Condition="'$(ProjectLanguage)' == 'CSharp' And '$(TargetFrameworkIdentifier)' != '.NETPortable' And '$(TargetCoreClr)' != 'True'" />
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" Condition="'$(ProjectLanguage)' == 'C++'" />
 
+  <PropertyGroup Condition="'$(OS)' != 'Windows_NT'">
+    <FinalDefineConstants>$(FinalDefineConstants.Replace('"', ''))</FinalDefineConstants>
+  </PropertyGroup>
+
   <PropertyGroup>
     <BuildDependsOn>
       $(BuildDependsOn);