I think this missing dep is what broke my parallel build
I used make -j2 with ghc-6.4.2.20060323 and got:

------------------------------------------------------------------------
==fptools== make boot -wr --jobserver-fds=3,11 -j;
in /var/tmp/portage/ghc-6.4.2_pre20060323/work/ghc-6.4.2.20060323/ghc/includes
------------------------------------------------------------------------
Creating ghcplatform.h...
Done.
gcc -O -O2 -march=k8 -pipe -Wa,--noexecstack    -c mkDerivedConstants.c -o mkDerivedConstants.o
In file included from ghcconfig.h:5,
                 from Stg.h:42,
                 from Rts.h:19,
                 from mkDerivedConstants.c:20:
ghcplatform.h:1:1: unterminated #ifndef
Done.
						    
With this patch applied I can no longer repoduce this build bug.
So I think this patch should be applied to the cvs ghc-6-4-branch too.