Makefile: default pager on AIX to "more"

AIX doesn't ship with "less" by default, and their "more" is more featureful than average, so the latter is a more sensible choice. People who really want less can set the compile-time option themselves, or users can set $PAGER. Signed-off-by: Jeff King <peff@peff.net> Tested-by: Tor Arntsen <tor@spacetec.no> Signed-off-by: Junio C Hamano <gitster@pobox.com>