Makefile: check if librt is an external library

Some systems, e.g. OpenBSD, have clock_gettime / librt build into the
libc and therefore fail to link against it with -lrt. Thus, detect this
and link only if needed.

Change-Id: I2c1668a350aa0806fccfb4e9cd8b04861f085ee9
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/61523
Original-Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72291
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
2 files changed