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 <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-sh-setup.sh

@@ -160,9 +160,11 @@ git_pager() {
 	else
 		GIT_PAGER=cat
 	fi
-	: "${LESS=-FRX}"
-	: "${LV=-c}"
-	export LESS LV
+	for vardef in @@PAGER_ENV@@
+	do
+		var=${vardef%%=*}
+		eval ": \"\${$vardef}\" && export $var"
+	done
 
 	eval "$GIT_PAGER" '"$@"'
 }