Support GCC 4.6's broken wait_for() signature.

This #ifdef is needed to compile with GCC 4.6.  We don't intend to support 4.6,
but for now I need this change to keep developing on Precise machines.  On Precise, you
can pull in a backport of GCC 4.7, but then you run into a Boost 1.46 bug that
make the shared_ptr not compile.  And then you need a newer Boost, at which
point, you would need to compile pretty much everything from source.