Comment the target Ruby variants

Author: nurse

ext/mysql2/statement.c

@@ -231,11 +231,14 @@ static int my_big2ll(VALUE bignum, LONG_LONG *ptr)
         nlz_bits == 0 &&
 #endif
 #if defined(HAVE_RB_ABSINT_SIZE) && defined(HAVE_RB_ABSINT_SINGLEBIT_P)
-        /* only -0x8000000000000000 is safe if `len == 8 && nlz_bits == 0` */
+        /* Optimized to avoid object allocation for Ruby 2.1+
+         * only -0x8000000000000000 is safe if `len == 8 && nlz_bits == 0`
+         */
         !rb_absint_singlebit_p(bignum)
 #elif defined(HAVE_RB_BIG_CMP)
         rb_big_cmp(bignum, LL2NUM(LLONG_MIN)) == INT2FIX(-1)
 #else
+        /* Ruby 1.8.7 doesn't have rb_big_cmp */
         rb_funcall(bignum, intern_cmp, 1, LL2NUM(LLONG_MIN)) == INT2FIX(-1)
 #endif
        ) {