configure: bugfix in check_boost_thread_libs()

Hi Jae-hyeon,

sorry to spam you, but I think there is a tiny bug in your recent
configure script commit.  You wrote

  if [ -z "x$found_lib" ]; then

This condition will never be true, because of the `x' in the string.  :)

Best,
Alex
Merge branch 'master' of ssh://pktlogin.phy.tu-dresden.de/soft/users/theory/FlexibleSUSY
convert test file into unix format