Depend on lang/gccXY if users wish to use a different version of gcc by
default than lang/gcc (currently 4.8).

(I don't fully agree with this implementation but this makes something
like DEFAULT_VERSIONS+=gcc=4.9 in make.conf work correctly.)

Reported by: Luca Pizzamiglio <luca.pizzamiglio@gmail.com>
Approved by: gerald