Try to fix a highly parallel build of libcmis with a non-system curl

The problem us that gb_LinkTarget__use_curl only declared a dependency
on an *unpacked* curl tarball, but the curlbuild.h file is *generated*
during curl configury. So something that depends on a (non-system)
curl needs to depend on curl having been configured at least. Let's
try like this.

Change-Id: I87b2a3292807d9bb873c3656caf58c4d98d8f622

RepositoryExternal.mk

@@ -1624,7 +1624,7 @@ $(eval $(call gb_Helper_register_libraries,PLAINLIBS_OOO, \
 ))
 
 define gb_LinkTarget__use_curl
-$(call gb_LinkTarget_use_unpacked,$(1),curl)
+$(call gb_LinkTarget_use_external_project,$(1),curl)
 $(call gb_LinkTarget_set_include,$(1),\
 	-I$(call gb_UnpackedTarball_get_dir,curl/include) \
 	$$(INCLUDE) \