Change of ADMB default options

As of ADMB 11.1, compiling with safe bounds is the default. This means R2admb will always compile with safe bounds. For my local build, I have changed line 45 in base-funs.r from

```
if (safe) args <- paste(args,"-s")
```

to

```
if (safe) args <- paste(args,"-s") else args <- paste(args, "-O")
```

but I'm not sure this is backwards compatible. I think the option -O is only legal with version 11.1, and so an error will get thrown if the user is not using ADMB 11.1 (although I have not tested this).

Start at line 45 of base-funs.r and inspect how the safe option builds ADMB compiler arguments. Check the behavior with ADMB 11.1 and older versions, including whether -O is accepted, then verify that R2admb compiles correctly for each supported version.

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