Use ruby functions if RUBY is defined

Author: nobu

st.c

@@ -342,7 +342,7 @@ get_power2(st_index_t size)
     unsigned int n = ST_INDEX_BITS - nlz_intptr(size);
     if (n <= MAX_POWER2)
         return n < MINIMAL_POWER2 ? MINIMAL_POWER2 : n;
-#ifndef NOT_RUBY
+#ifdef RUBY
     /* Ran out of the table entries */
     rb_raise(rb_eRuntimeError, "st_table too big");
 #endif