Current best practice to select GCC compiler in Portfile

[Sean Farley]

David Strubbe writes:

> Hi Rainer,
>
> Sean did indeed add the default_variants to the compilers portgroup in the
> last month or two. Perhaps you were trying before then.

Thanks for following up, David :-)

@Rainer, yes, it should definitely handle default_variants. Please let
me know otherwise (preferably with a reproducible test case!).