Change occurrences of gcc to the more general cc

This is done for the sake of Yosemite, which does not have gcc, and yet
this change is also compatible with Linux.

This is the first in an eventual series of PRs resulting from the `##nix-darwin` work to make darwin in nixpkgs significantly more robust. This is the only change that is expected to require major rebuilds on Linux (though more darwin rebuilds can be expected moving forward).