build: make git submodule checking more reliable

* cfg.mk (_curr_status): Accommodate leading "+" or "-" in the output
of git submodule status.
* autogen.sh: Likewise.

autogen.sh

@@ -65,7 +65,7 @@ fi
 # Ensure that whenever we pull in a gnulib update or otherwise change to a
 # different version (i.e., when switching branches), we also rerun ./bootstrap.
 curr_status=.git-module-status
-t=$(git submodule status)
+t=$(git submodule status|sed 's/^[ +-]//;s/ .*//')
 if test "$t" = "$(cat $curr_status 2>/dev/null)"; then
     : # good, it's up to date, all we need is autoreconf
     autoreconf -if