USES=compiler: nested functions, GCC preference.

[Matthias Andree]

Baptiste,

I maintain a few ports that require a compiler that supports nested
functions in C. This means "GCC" because to my knowledge no clang
version supports this, and the 3.4-related docs still state unsupported
and unlikely.

Do you think it worthwhile to extend USES=compiler so that a port can
request "nested functions"?  This would become relevant if USE_GCC were
to be removed later on in the game.

Thanks for considering this.

Best regards
Matthias