Now gcc is just another build input, making it possible in the future
to have a stdenv that doesn't depend on a C compiler. This is very
useful on NixOS, since it would allow trivial builders like
writeTextFile to work without pulling in the C compiler.
This is useful for non-Autoconf-based packages, since GNU Make's
default for CXX is "g++". (The CC default is "cc" so should work fine
with Clang already.)
