make failure trying to build gmp-5.1.0a on Tru64unix v5.1b system

[Torbjorn Granlund]

bodrato at mail.dm.unipi.it writes:

  In both functions, we already have a copy of dp[dn-1] in another variable.
  
  Can you please try the attached patch?
  
OK, but we must realise that we will never be able to keep GMP working
if a compiler (and/or its include files) has this sort of bugs.

HP also terminated support for OSF/1 a while ago, so no fixes can be
expected.

-- 
Torbjörn