Makefile: Do not create empty autoconf.mk on error

The build rules of
  - include/autoconf.mk.dep
  - include/autoconf.mk
  - include/spl-autoconf.mk
  - include/tpl-autoconf.mk
were not nice.

They created empty files (which are never updated)
if an error occurs during preprocessing.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>