gcc: Fix purity

Somewhere the no-sys-dirs.patch got disabled, so gcc was looking in
/usr/local/include and /usr/lib. Since I can't fix the patch easily,
I've borrowed the --sysroot trick from clang-wrapper. This causes
builtin paths to be prefixed with /var/empty
(e.g. /var/empty/usr/lib), which don't exist.

pkgs/development/compilers/gcc/4.8/default.nix

@@ -67,7 +67,6 @@ let version = "4.8.3";
     patches = []
       ++ optional enableParallelBuilding ./parallel-bconfig.patch
       ++ optional (cross != null) ./libstdc++-target.patch
-      # ++ optional noSysDirs ./no-sys-dirs.patch
       # The GNAT Makefiles did not pay attention to CFLAGS_FOR_TARGET for its
       # target libraries and tools.
       ++ optional langAda ./gnat-cflags.patch