Correctly detect warning flags with clang

Correctly detect warning flags with clang

Clang will happily claim to support any warning flags
unless the -Werror and -Wunknown-warning-option flags
are set. Thus we need to make sure these are set when
testing for clags.

We must also set the clang specific warning flags
-Wno-unused-command-line-argument to avoid a warning
from the ssp-buffer-size flag when linking .o files.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>