postprocess: only check-dynamic-objects if instset is created

People who use --with-system-libs will derive no benefit from
check-elf-dynamic-objects.  Since checking all SYSTEM_* variables
seems unmaintainable, check if an instset is to be created as a
heuristic that the build should be portable and libraries checked.
This should run with no config changes at least on the tinderboxes;
Jenkins builders don't create instsets.

Change-Id: I2407429891210ea79917fc4d01a933ec661325b1

postprocess/Module_postprocess.mk

@@ -42,10 +42,12 @@ $(eval $(call gb_Module_add_targets,postprocess, \
 endif
 
 ifeq ($(OS),LINUX)
+ifneq ($(PKGFORMAT),)
 $(eval $(call gb_Module_add_check_targets,postprocess,\
 	CustomTarget_check_dynamic_objects \
 ))
 endif
+endif
 
 $(eval $(call gb_Module_add_check_targets,postprocess,\
 	CppunitTest_services \