build: get rid of INCLUDES, use AM_CPPFLAGS

build: get rid of INCLUDES, use AM_CPPFLAGS

INCLUDES in configure.ac was not used at all, and INCLUDES in
Makefile.am is supposed to be AM_CPPFLAGS these days.

Reduces warnings spewed during bootstrap/autoreconf.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Greg Troxel <gdt@ir.bbn.com>
Acked-by: Feng Lu <lu.feng@6wind.com>
Acked-by: Paul Jakma <paul@jakma.org>
(cherry picked from commit 237aac56960575f6ad2451ba2796d94bd5ae4b33)