Suppress the alarming SpecConstr message for normal users (Trac #5125)

This is the offending message:
  SpecConstr
      Function `$wks2{v s2dJ} [lid]'
        has one call pattern, but the limit is 0
      Use -fspec-constr-count=n to set the bound
      Use -dppr-debug to see specialisations

The message isn't very good, and is for experts only. So now it
comes out only
    if you build with -DDEBUG
    or you specify -dppr-debug at runtime