make add_custom_headers better

solenv/gbuild/LinkTarget.mk

@@ -907,7 +907,7 @@ $(2) :|	$(call gb_LinkTarget_get_external_headers_target,$(1))
 
 endef
 
-define gb_LinkTarget_add_custom_headers
+define gb_LinkTarget__add_custom_headers
 $(call gb_LinkTarget_get_headers_target,$(1)) \
 $(call gb_LinkTarget_get_target,$(1)) : INCLUDE += -I$(call gb_CustomTarget_get_workdir,$(2))
 ifeq ($(gb_FULLDEPS),$(true))
@@ -918,6 +918,11 @@ $(call gb_LinkTarget_get_clean_target,$(1)) : $(call gb_CustomTarget_get_clean_t
 
 endef
 
+define gb_LinkTarget_add_custom_headers
+$(foreach customtarget,$(2),$(call gb_LinkTarget__add_custom_headers,$(1),$(customtarget)))
+
+endef
+
 define gb_LinkTarget_add_package_headers
 $(foreach package,$(2),$(call gb_LinkTarget__add_internal_headers,$(1),$(call gb_Package_get_target,$(package))))
 $(call gb_LinkTarget_get_clean_target,$(1)) : $(foreach package,$(2),$(call gb_Package_get_clean_target,$(package)))