buildsys: fix static configuration and building

buildsys: fix static configuration and building

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/

Signed-off-by: Lada Trimasova <ltrimas@synopsys.com>