The differences will be in your .proj file.
Having tried it myself you will have to do all of the following;
- Remove the
<TargetFrameworkProfile>
element - Remove the
<ProjectTypeGuids>
element - Change where you have #2 (below) for what I shown in #1
1. Regular class library
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
2. Portable class library
<Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />