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.

pkgs/os-specific/linux/systemd/default.nix

@@ -86,7 +86,7 @@ stdenv.mkDerivation rec {
 
   # This is needed because systemd uses the gold linker, which doesn't
   # yet have the wrapper script to add rpath flags automatically.
-  NIX_LDFLAGS = "-rpath ${pam}/lib -rpath ${libcap}/lib -rpath ${acl}/lib -rpath ${stdenv.gcc.gcc}/lib";
+  NIX_LDFLAGS = "-rpath ${pam}/lib -rpath ${libcap}/lib -rpath ${acl}/lib -rpath ${stdenv.cc.gcc}/lib";
 
   PYTHON_BINARY = "${coreutils}/bin/env python"; # don't want a build time dependency on Python