Force teamcity to look for Mono dlls but not to GAC

Hi,

I would like to ask about building Mono solution with MSBuild task. The thing is that we are developing projects with Xamarin studio and plugin for VS. The csproj are configured to use MSBuild for MonoDroid libraries and building successfully with Visual Studio or msbuild command via command prompt.
At the same point of time, I'm unable to build the solution with TeamCity because I don't know how to configure looking for the assemblies not to GAC and Framework/4.0... but for MonoDroid libraries installed in the same machine. During build phase I see an errors like
- 'The type or namespace name 'Mono' could not be found' or  
- The type 'System.Action<T1,T2>' exists in both 'c:\Windows\Microsoft.NET\Framework64\v4.0.30319\mscorlib.dll' and 'c:\Windows\assembly\GAC_MSIL\System.Core\3.5.0.0__b77a5c561934e089\System.Core.dll'

Do you know how to force TeamCity to look into Mono libraries instead of GAC? Again, as I said earlier, the project is configured properly and I'm able to build solution with msbuild command without TeamCity

Thanks,
Alex