[gfortran] Support INTEGER<->LOGICAL conversion as GNU extension

[FX Coudert]

> It's just that in this case, the recommendation appears
> to conflict with existing practice in the gfortran front-end.  I do
> admit that g77 was just as inconsistent requiring -fugly-int.

I think the main point of gfortran when it comes to options is to 
default to the g77 behavior. I'd go for that too (not that my opinion 
has much importance ;-)

FX