configure: do not rerun the tests with -Werror

Tests run in configure are pretty trivial at this point, so
do not bother with the extra complication of running tests
both with and without -Werror.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>