No need to define ROSLYN_4_12 when in source build

Author: JoeRobich

src/RoslynAnalyzers/Directory.Build.props

@@ -8,8 +8,11 @@
 
     <!-- Set 'NoDefaultExcludes' to ensure that we can package .editorconfig files into our analyzer NuGet packages -->
     <NoDefaultExcludes>true</NoDefaultExcludes>
-    
+
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(DotNetBuildSourceOnly)' != 'true'">
     <DefineConstants>$(DefineConstants),ROSLYN_4_12_OR_LOWER</DefineConstants>
   </PropertyGroup>