Configure: for '-z defs', also check $config{cflags}

When sanitize options are added as 'enable-msan' or similar, the
-fsanitize C flags is set in $config{cflags} rather than
$config{CFLAGS}, so we need to check both.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8965)