gb_LinkTarget_get_preparation_target is not defined (2nd try)

gb_ExternalProject_use_unpacked has completely different different
side-effects, so use the manual way/the actually expected part of
the rule

Change-Id: I35edb19c3e768608bbb70314cac7ca908a426c7c
Reviewed-on: https://gerrit.libreoffice.org/1067Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

RepositoryExternal.mk

@@ -454,12 +454,12 @@ $(call gb_LinkTarget_use_static_libraries,$(1),\
 endef
 
 define gb_LinkTarget__use_boost_headers
-$(call gb_LinkTarget_get_preparation_target,$(1)) : $(call gb_UnpackedTarball_get_target,boost)
+$(call gb_LinkTarget_use_unpacked,$(1),boost)
 
 endef
 
 define gb_ExternalProject__use_boost_headers
-$(call gb_ExternalProject_get_preparation_target,$(1)) : $(call gb_UnpackedTarball_get_target,boost)
+$(call gb_ExternalProject_get_preparation_target,$(1)) :| $(call gb_UnpackedTarball_get_final_target,boost)
 
 endef
 endif # SYSTEM_BOOST