configure: Add -Werror to avx2, avx512 tests

When using clang, we get

ERROR: configure test passed without -Werror but failed with -Werror.
       This is probably a bug in the configure script. The failing command
       will be at the bottom of config.log.
       You can run configure with --disable-werror to bypass this check.

What we really want from these two tests is whether the
entire code sequence is supported, including pragmas.
Adding -Werror makes the test properly fail for clang.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210719200112.295316-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -3881,7 +3881,7 @@ static int bar(void *a) {
 }
 int main(int argc, char *argv[]) { return bar(argv[0]); }
 EOF
-  if compile_object "" ; then
+  if compile_object "-Werror" ; then
     avx2_opt="yes"
   else
     avx2_opt="no"
@@ -3911,7 +3911,7 @@ int main(int argc, char *argv[])
 	return bar(argv[0]);
 }
 EOF
-  if ! compile_object "" ; then
+  if ! compile_object "-Werror" ; then
     avx512f_opt="no"
   fi
 else