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>
diff --git a/config.mak.uname b/config.mak.uname
index a88f139..9b73c66 100644
--- a/config.mak.uname
+++ b/config.mak.uname
@@ -206,6 +206,7 @@
 	HAVE_PATHS_H = YesPlease
 	GMTIME_UNRELIABLE_ERRORS = UnfortunatelyYes
 	HAVE_BSD_SYSCTL = YesPlease
+	PAGER_ENV = LESS=FRX LV=-c MORE=FRX
 endif
 ifeq ($(uname_S),OpenBSD)
 	NO_STRCASESTR = YesPlease