Thanks to all who replied both on and offline. I don't think there is any argument about what is meant. My issue was syntactic, not semantic. As far as can see, icchaṃ is the present participle of Icchati, not a noun; labhati is a 3rd, sing., present indicative verb. So, as far as I can see, the sentence means

   Desiring that which [one] does not obtain [is] dukkha.