If there is a warning similar to:
/lib/mono/4.5/Microsoft.Common.targets: warning : Unable to find framework corresponding to the target framework moniker '.NETFramework,Version=v2.0'. Framework assembly references will be resolved from the GAC, which might not be the intended behavior.
You can override the .NET Framework on the command line:
xbuild /p:TargetFrameworkVersion="v4.0"
You can change your solution:
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>