build.proj

<Project DefaultTargets="build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" >
    <PropertyGroup>
        <Configuration Condition="'$(Configuration)'==''">Release</Configuration>
    </PropertyGroup>
    <ItemGroup>
        <ProjectReferences Include="src\*.*proj" />
    </ItemGroup>
    <Target Name="build">
        <MSBuild Projects="@(ProjectReferences)"
            Targets="Build"
            Properties="Configuration=$(Configuration)" />
    </Target>
</Project>