set GCC_REQD based on USE_LANGUAGES (was: default C versions)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

>   Someone should prototype adding GCC_REQD automatically based on
>   USE_LANGUAGES, if PKGSRC_COMPILER contains (NB: not "is equal to")
>   gcc, and see how that works.  If it's as good as hand-sprinkled
>   GCC_REQD we should start removing the hand-sprinkled ones, which are a
>   kludge because it doesn't work automatically.
I did that a few years ago.