clang -nostdinc generates spurious unuused command line argument warnings

[yshui]

commit 7047ba9 removed a clang patch and uses the -nostdlibinc command line flag instead. this is mostly fine, and unnoticeable from the user's perspective.

except when the user passes -nostdinc also. without this change, clang won't complain; but now clang prints:

clang: warning: argument unused during compilation: '-nostdlibinc' [-Wunused-command-line-argument]

this breaks packages that are built with -Werror, such as the linux kernel.