define Package/xorg-lib1
[...]
endef

define Package/xorg-lib2
[...]
endef

SUBPACKAGES:=xorg-lib1 xorg-lib2
ifeq ($(SUBPACKAGE),)
  define Build/Compile
    $(foreach SUBPACKAGE,$(SUBPACKAGES),$(NO_TRACE_MAKE) SUBPACKAGE=$(SUBPACKAGE);)
  endef
else
  define Build/Compile
  [... main compile commands ...]
  endef
endif
[ das gleiche nochmal für Build/Configure ]

$(eval $(call BuildPackage,xorg-lib1))
$(eval $(call BuildPackage,xorg-lib2))