1) autoconf is added in set_sys
2) building of linux header and glibc header is moved backward to build_crosss_gcc,
   cause they need the boot compiler to compile them.
   after all, what we need is the header for ARM platform.
3) all headers are installed into 'include' folder and the search path of other components are also moved to 'include'.
   A problem is that the final gcc still searches headers under 'usr/include', so I have to create a sym link.