Fixed a problem in the detection of Boost 1.50 (Sleep.cpp).

From: Marco Clemencic <marco.clemencic@cern.ch>


git-svn-id: file:///afs/cern.ch/work/b/binet/dev/repos/gaudi-svn-mirror/Gaudi/trunk/GaudiKernel@7121 22b0f739-a4da-4a50-9948-337547c35a65