gb_LinkTarget_get_preparation_target is not defined

and in turn build breaks because boost headers are not ready.
use the suggestion by mst and use gb_LinkTarget_use_unpacked instead.
Also use gb_ExternalProject_use_unpacked for consistency

Change-Id: Ie52ba7cd99ae6622b11f32eca8895ca004b8ae95
Reviewed-on: https://gerrit.libreoffice.org/1066Reviewed-by: Tor Lillqvist <tml@iki.fi>
Tested-by: Tor Lillqvist <tml@iki.fi>

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_use_unpacked,$(1),boost)
 
 endef
 endif # SYSTEM_BOOST