use replacement functions if they're available

we use the fact that in the same move of deprecating those functions,
tommath_class.h also isn't included anymore in tommath.h so those defines
are gone as well and can be used as an indication if the new functions are
available

src/math/ltm_desc.c

@@ -89,13 +89,22 @@ static int init_copy(void **a, void *b)
 static int set_int(void *a, ltc_mp_digit b)
 {
    LTC_ARGCHK(a != NULL);
+#ifdef BN_MP_SET_INT_C
    return mpi_to_ltc_error(mp_set_int(a, b));
+#else
+   mp_set_u32(a, b);
+   return CRYPT_OK;
+#endif
 }
 
 static unsigned long get_int(void *a)
 {
    LTC_ARGCHK(a != NULL);
+#ifdef BN_MP_GET_INT_C
    return mp_get_int(a);
+#else
+   return mp_get_ul(a);
+#endif
 }
 
 static ltc_mp_digit get_digit(void *a, int n)