pager: move pager-specific setup into the build

pager: move pager-specific setup into the build

Allowing PAGER_ENV to be set at build-time allows us to move
pager-specific knowledge out of our build.  This allows us to
set a better default for FreeBSD more(1), which pretends not to
understand ANSI color escapes if the MORE environment variable
is left empty, but accepts the same variables as less(1)

Originally-from:
 https://public-inbox.org/git/xmqq61piw4yf.fsf@gitster.dls.corp.google.com/

Helped-by: Junio C Hamano <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>