Do not activate strlen assume for debug mode.

This will make debug mode faster.

It also fixes a problem with GCC7 which makes
**release**mode slower (since it called strlen!)

tests/test-mstring.c

@@ -21,6 +21,7 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
 
+#define STRING_WITHIN_TEST
 #include "m-string.h"
 
 int main(void)