fix 'jobserver unavailable: using -j1' warning for $(SUBMAKE)

- Problem is that make doesn't recognize $(SUBMAKE) as
  make, so it won't pass MAKEFLAGS and doesn't use it's
  jobserver.

- Fixed by explicitly marking those lines as make using
  the leading '+' token.

For reference:
http://www.gnu.org/software/make/manual/html_node/Error-Messages.html
http://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html#MAKE-Variable