WITH
SET RealBuildTypes AS 
    {[Build].[Build Type].[Build Type].Members - [Build].[Build Type].UNKNOWNMEMBER}


SELECT NON EMPTY
{
   [Measures].[Build Details Count] 
} 
ON COLUMNS
,NON EMPTY{
  [Team Project].[Team Project].[Team Project].Members * RealBuildTypes  
} ON ROWS
FROM [Team System]