For now, we don't NATIVE_SYSTEM_HEADER_DIR because it breaks the
build. However, it points to Glibc in the Nix store (not /usr/include)
so it's kind of okay.
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.