RFA: Set default optimization level to -O2 for -flto

[Nick Clifton]

Hi Basile,

> but my personal feeling is
> that make a warning -or perhaps even an error- at link time if the
> optimization is less than at compile time.

I think that a warning would be better than an error because the user 
might deliberately wish to use a lower optimization level.  (For example 
because they suspect that there is a bug in the optimizer and they are 
trying to prove this).

Also I think that it would be best to only issue the warning if *no* 
-O<n> or -Os switch is specified on the command line.  This is again 
because the user may have a good reason for selecting a low optimization 
level.

Cheers
   Nick