problem with -o and header file names.

[P.C.Callaghan]

When compiling with a line like this:

> ghc-4.03 -syslib posix -syslib exts -dshow-passes -ddump-spec
> -fwarn-incomplete-patterns -fwarn-unused-binds -hi-diffs -cpp -H60M -K4M
> -Onot -c -o X.o X.a.hs

 - where the intention is that module "X" from file X.a.hs
should be compiled in to X.o, the header file created gets the
wrong name (the module id is duplicated): 

> X.X.hi

Paul