parallel builds

[Duncan Coutts]

With last night's ghc 6.4.2 snapshot I still need this patch applied to
fix building with make -j2

http://www.haskell.org//pipermail/cvs-ghc/2006-March/028982.html
"mkDerivedConstants.c depends on ghcplatform.h"

I think it should go to the STABLE ghc branch as well as HEAD.

I've had successful parallel builds on several machines (x86, amd64,
ia64) with this patch and none without.

Duncan