Fix detection of supported warnings in clang

Fix detection of supported warnings in clang

Without -Werror=unknown-warning-option, clang will warn for
unrecognized -W options like "-Werror=discarded-qualifiers" but won't
return a nonzero exit status, leading configure to think the options
are supported and thus include them during the build, leading to a
rather noisy log.

This option isn't needed during the build, though it won't hurt
anything either.  It is desirable during the testing of other -W
options for cleaner results, but the existing code tests each option
independently, requiring different handling for this option than for
other -W options.