[patch] fix cflags check when using clang compiler

[and]

fix cflags check when using clang compiler

with clang invalid cflags usage result in exit 0, we expect exit 1

$ clang -c -Wmaybe-uninitialized conftest.c ; echo $?
warning: unknown warning option '-Wmaybe-uninitialized'; did you mean '-Wuninitialized'? [-Wunknown-warning-option]
1 warning generated.
0

solution with -Werror on cflags check

$ clang -c -Werror -Wmaybe-uninitialized conftest.c ; echo $?
error: unknown warning option '-Wmaybe-uninitialized'; did you mean '-Wuninitialized'? [-Werror,-Wunknown-warning-option]
1

(tested with clang 3.6)

Signed-off-by: Andreas Mohr <and@…>

[andrew_b]

Thanks!
Applied as [495b4d662f23ab87c83d4f49386487a43251370c].