usage message

[Stephen Weeks]

> I agree that there are two calls is not obvious.  I guess the right thing to do
> is to break all the calls and insert backslashes.

I did this and cleaned up some of the args passed to gcc.  Here's what mlton -v
now looks like.

% mlton -v /tmp/z.sml
Compile SML starting
      gcc -DNODEBUG -DMLton_safe=TRUE -DMLton_detectOverflow=TRUE \
          -I/home/sweeks/mlton/include -o /tmp/filem3vdgo /tmp/fileVd9eeh.c \
          -L/home/sweeks/mlton/lib -lmlton -lm -lgmp
      /tmp/filem3vdgo /tmp/file4JPkHC
Compile SML finished in 2.760
Compile C starting
   gcc -S -DNODEBUG -DMLton_safe=TRUE -DMLton_detectOverflow=TRUE \
       -I/home/sweeks/mlton/include -o /tmp/file2rtabe.s /tmp/fileSA70p2.c
Compile C finished in 0.190
Assemble starting
   gcc -c -o /tmp/fileICBxrd.o /tmp/file2rtabe.s
   gcc -c -o /tmp/fileUTuFMd.o /tmp/file41BBjr.0.S
Assemble finished in 0.040
Link starting
   gcc -o /tmp/z /tmp/fileICBxrd.o /tmp/fileUTuFMd.o -L/home/sweeks/mlton/lib \
       -lmlton -lm -lgmp
Link finished in 0.120