build: avoid autogen on 'make clean'

Tested by running 'git submodule foreach git pull origin master' and
'git add .gnulib', then seeing that 'make clean' skips autogen
although 'make' properly runs it.

* cfg.mk (_clean_requested): New check, to speed up 'make clean'
even if gnulib submodule is outdated.
Suggested by Daniel P. Berrange.

cfg.mk

@@ -301,7 +301,8 @@ ifeq (0,$(MAKELEVEL))
 		git diff .gnulib);					\
       stamp="$$($(_submodule_hash) $(_curr_status) 2>/dev/null)";	\
       test "$$stamp" = "$$actual"; echo $$?)
-  ifeq (1,$(_update_required))
+  _clean_requested = $(filter %clean,$(MAKECMDGOALS))
+  ifeq (1,$(_update_required)$(_clean_requested))
     $(info INFO: gnulib update required; running ./autogen.sh first)
 Makefile: _autogen
   endif