Allow building bytecode-only on native archs.

Index: src/Makefile
--- src/Makefile.orig
+++ src/Makefile
@@ -33,13 +33,17 @@ COBJS=bng.$(O) nat_stubs.$(O)
 
 all:: libnums.$(A) nums.cma
 
+ifeq "$(NATIVE)" "true"
 ifneq "$(ARCH)" "none"
 all:: nums.cmxa
 endif
+endif
 
+ifeq "$(NATIVE)" "true"
 ifeq "$(NATDYNLINK)" "true"
 all:: nums.cmxs
 endif
+endif
 
 libnums.$(A): $(COBJS)
 	$(OCAMLMKLIB) -oc nums $(COBJS)
@@ -71,11 +75,15 @@ nat_stubs.$(O): bng.h nat.h
 # is installed via findlib
 
 TOINSTALL=nums.cma libnums.$(A) $(CMIS) $(CMIS:.cmi=.mli) $(CMIS:.cmi=.cmti)
+ifeq "$(NATIVE)" "true"
 ifneq "$(ARCH)" "none"
 TOINSTALL+=nums.cmxa nums.$(A) $(CMXS)
 endif
+endif
+ifeq "$(NATIVE)" "true"
 ifeq "$(NATDYNLINK)" "true"
 TOINSTALL+=nums.cmxs
+endif
 endif
 ifeq "$(SUPPORTS_SHARED_LIBRARIES)" "true"
 TOINSTALL_STUBS=dllnums$(EXT_DLL)