how to modify the number of parallel compilation with Xcode

Question

How to change the number of parallel compilations when building a C++ project with XCode ?

I am not talking about distributed builds, but on simultaneous compilations of

Answer 1

Just for the record - developers at our site using XCode 4 discovered that parallel compilation gets disabled if McAfee anti-virus is installed. Builds that should finish in 15-20 minutes took nearly five hours until McAfee was removed.