Add RoslynIncompatibleMsbuildVersion condition on Roslyn import

Due to merge conflict this condition got removed so adding it back
to allow for older msbuild versions to work (at least temporarily).

Author: weshaggard

tests/dir.props

@@ -78,8 +78,8 @@
   </PropertyGroup>
 
   <!-- Use Roslyn Compilers to build -->
-  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'!='true' and Exists('$(RoslynPropsFile)') and '$(UseRoslynCompilers)'!='false'" />
-  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'=='true' and Exists('$(RoslynPropsFile)')" />
+  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'!='true' and Exists('$(RoslynPropsFile)') and '$(UseRoslynCompilers)'!='false' and '$(RoslynIncompatibleMsbuildVersion)' != 'true'" />
+  <Import Project="$(RoslynPropsFile)" Condition="'$(RunningOnUnix)'=='true' and Exists('$(RoslynPropsFile)') and '$(RoslynIncompatibleMsbuildVersion)' != 'true'" />
 
   <PropertyGroup Condition="'$(BuildAllProjects)'=='true'">
     <!-- When we do a traversal build we get all packages up front, don't restore them again -->