gcc: Add build->build dep for C compilers

5 already had this, but I forgot the other versions.

pkgs/development/compilers/gcc/6/default.nix

@@ -276,6 +276,7 @@ stdenv.mkDerivation ({
   inherit noSysDirs staticCompiler langJava
     libcCross crossMingw;
 
+  depsBuildBuild = [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ texinfo which gettext ]
     ++ (optional (perl != null) perl)
     ++ (optional javaAwtGtk pkgconfig);