libhaclml.cmxa: pass -thread to ocamlopt

This avoids a warning:

    ocamlfind: [WARNING] Package `threads': Linking problems may arise because of the missing -thread or -vmthread switch

Makefile

@@ -1034,7 +1034,7 @@ obj/libhaclml.cmxa: $(filter-out $(HACL_HOME)/obj/Meta_Interface.cmx,$(ALL_CMX_F
 # JP: doesn't work because a PPX is prepended for some reason
 # ocamlfind mklib -o haclml -package fstar.lib -g -I $(HACL_HOME)/obj $(addprefix $(HACL_HOME)/obj/*.,cmo cmx ml o)
 	$(call run-with-log,\
-	  OCAMLFIND_IGNORE_DUPS_IN="`ocamlc -where`/compiler-libs" ocamlfind opt -a -o $@ -package fstar.lib -g -I $(HACL_HOME)/obj $^ \
+	  OCAMLFIND_IGNORE_DUPS_IN="`ocamlc -where`/compiler-libs" ocamlfind opt -thread -a -o $@ -package fstar.lib -g -I $(HACL_HOME)/obj $^ \
 	  ,[OCAMLOPT-CMXA] libhaclml,$(call to-obj-dir,$@))