Exit cleanly if llvm-config cannot be found.
Fixed building host code (make menuconfig code). llvm-gcc out of
the box might not be able to link correctly so in this case we
use 'cc' instead of llvm-gcc for the host code.
configure script checks for ncurses (needed for make menuconfig)
Exit gracefully if argparse can't be imported (user probably has python
version < 2.7).