maint: Prefer AM_CPPFLAGS over INCLUDES

Our use of INCLUDES in Makefile.am hearkens back to when we had to
cater to automake 1.9.6 (thanks, RHEL 5) which lacked AM_CPPFLAGS.
Modern Automake flags a warning that INCLUDES is deprecated, and
now that we mandate RHEL 7 or better (see commit c1bc9c66), we no
longer have to cater to the old spelling. This change will also
make it easier to do per-binary CPPFLAGS.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>