util-linux: fix static builds with uClibc

In case of uClibc librt depends on libpthread. In particular
timer_create() function uses pthread_XXX(). That means in case
of static builds it's required to link not librt alone but
together with libpthread. So if checking timer_create function
in librt fails, it is necessary to check if timer_create function
successfully links with "-lpthread".

That issues was spotted in Buldroot autobuilder failures:
http://autobuild.buildroot.net/results/759/75960db671807091fe9155aee9e46a6245e32590/
http://autobuild.buildroot.org/results/112/112e8b85783f5aaba42a937a6eb064317615a21b/

0005-build-sys-use-REALTIME_LIBS.patch is a back-port of upstream
https://github.com/karelzak/util-linux/commit/b97edfe731fdf3d3e92e40494b22658207ab6d3c
that won't apply cleanly on v2.27.1

0006-buildsys-fix-static-configuration-and-building.patch was just
applied upstream, see https://github.com/karelzak/util-linux/commit/71a77ca1ddfdd7e0d14caad9e8a607a83d61a45c



Both patches must be removed on util-linux version bump.

Signed-off-by: Lada Trimasova <ltrimas@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>