Fix boost::thread compilation on OS X Snow Leopard.
Fix boost::thread compilation on OS X Snow Leopard.

cgvarela/passenger,erikogan/passenger,openSUSE/passenger,clemensg/passenger,phusion/passenger,clemensg/passenger,clemensg/passenger,antek-drzewiecki/passenger,antek-drzewiecki/passenger,bf4/passenger,cgvarela/passenger,phusion/passenger,clemensg/passenger,kewaunited/passenger,jawj/passenger,fabiokung/passenger-debian,pkmiec/passenger,clemensg/passenger,cgvarela/passenger,pkmiec/passenger,phusion/passenger,cgvarela/passenger,kewaunited/passenger,kewaunited/passenger,cgvarela/passenger,phusion/passenger,gravitystorm/passenger,jawj/passenger,openSUSE/passenger,bf4/passenger,fabiokung/passenger-debian,kewaunited/passenger,fabiokung/passenger-debian,kewaunited/passenger,gravitystorm/passenger,pkmiec/passenger,erikogan/passenger,cgvarela/passenger,jawj/passenger,fabiokung/passenger-debian,antek-drzewiecki/passenger,antek-drzewiecki/passenger,cgvarela/passenger,jawj/passenger,bf4/passenger,antek-drzewiecki/passenger,openSUSE/passenger,bf4/passenger,antek-drzewiecki/passenger,openSUSE/passenger,bf4/passenger,openSUSE/passenger,openSUSE/passenger,phusion/passenger,gravitystorm/passenger,pkmiec/passenger,erikogan/passenger,fabiokung/passenger-debian,gravitystorm/passenger,antek-drzewiecki/passenger,gravitystorm/passenger,clemensg/passenger,phusion/passenger,phusion/passenger,phusion/passenger,jawj/passenger,kewaunited/passenger,antek-drzewiecki/passenger,erikogan/passenger,pkmiec/passenger,kewaunited/passenger,erikogan/passenger,pkmiec/passenger,erikogan/passenger,bf4/passenger,jawj/passenger,cgvarela/passenger,clemensg/passenger,kewaunited/passenger,pkmiec/passenger,clemensg/passenger,jawj/passenger,bf4/passenger,fabiokung/passenger-debian,gravitystorm/passenger