the mono 3.4 archive is missing a file named Microsoft.Portable.Common.targets which should be located at path mcs/tools/xbuild/targets/Microsoft.Portable.Common.targets
with the following content:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<Import Project="..\Microsoft.Portable.Core.props" />
<Import Project="..\Microsoft.Portable.Core.targets" />
</Project>
See this bug: https://bugzilla.xamarin.com/show_bug.cgi?id=18690