respect NDEBUG.

ruby/ruby.h includes ruby/assert.h, and RUBY_NDEBUG is defined
by checking NDEBUG. In other words, NDEBUG is only seen just
after ruby/ruby.h. This patch also cheks NDEBUG just after
including ruby_assert.h.

Without this patch, assertions in array.c are always enabled.

ruby_assert.h

@@ -8,3 +8,8 @@
 #undef assert
 #define assert RUBY_ASSERT
 #endif
+
+#ifdef NDEBUG
+  #undef  RUBY_NDEBUG
+  #define RUBY_NDEBUG 1
+#endif