Define MS_ASYNC and similar constants on Windows too
Otherwise the new base/docs/helpdb.jl cannot be built on Windows
This may not be the right fix, but it should work as a temporary fix.
Ref https://groups.google.com/forum/#!topic/julia-dev/OMi7TsNuQNg
for those of us who weren't in the same room while the new system
was put in place.
