make: ensure `base/Makefile.defs` can be included

Automatically run `fetchthirdparty`.

Makefile

@@ -1,9 +1,7 @@
 # koreader-base directory
 KOR_BASE?=base
 
-# the repository might not have been checked out yet, so make this
-# able to fail:
--include $(KOR_BASE)/Makefile.defs
+include $(KOR_BASE)/Makefile.defs
 
 # We want VERSION to carry the version of the KOReader main repo, not that of koreader-base
 VERSION:=$(shell git describe HEAD)
@@ -157,7 +155,7 @@ coverage: $(INSTALL_DIR)/koreader/.luacov
 		+$$(($$(grep -nm1 -e "^Summary$$" luacov.report.out|cut -d: -f1)-1)) \
 		luacov.report.out
 
-fetchthirdparty:
+$(KOR_BASE)/Makefile.defs fetchthirdparty:
 	git submodule init
 	git submodule sync
 ifneq (,$(CI))