build/clang: fix detection of valid warning compiler flags

build/clang: fix detection of valid warning compiler flags

clang does not exit with error when it is called with an
unrecognized (warning) option. Instead it just prints
a warning that makes the configure script believe the
warning is supported. Later, during build we might pass
-Werror, which causes clang to fail due to unrecognized
arguments.

Fix the script to detect compiler warnings by passing
'-Werror=unknown-warning-option', which lets clang fail too.

Signed-off-by: Thomas Haller <thaller@redhat.com>