boost: fix build error with recent versions of glibc

Original patch is at https://svn.boost.org/trac/boost/changeset/84950

Signed-off-by: Yvan Volochine <yvan.volochine@gmail.com>
(cherry picked from commit 7493ea56e90265440a4e639cf6d912b9a23c947c)

Signed-off-by: Dan Stowell <danstowell@users.sourceforge.net>