src: raise _XOPEN_SOURCE definition to 600

Following the guidelines from @bnoordhuis
This comment is not quite true, is it?  IIRC, O_NONBLOCK is always visible on the BSDs.  It should explain that >= 500 is required for Solaris (I think? O_NONBLOCK is never hidden on Darwin and Linux, either -  paging @indutny) but that it's raised to 600 to avoid hiding definitions for the include of uv.h further down below.