src: fix _XOPEN_SOURCE redefinition warning

Fix the following compiler warning on systems where _XOPEN_SOURCE is
defined by default:

    ../src/node_constants.cc:35:0: warning: "_XOPEN_SOURCE" redefined
     #define _XOPEN_SOURCE 500

Move the (re)definition of _XOPEN_SOURCE to the top of the file while
we're here.  Commit 00890e4 adds a `#define _XOPEN_SOURCE 500` in order
to make <fcntl.h> expose O_NONBLOCK but it does so after other system
headers have been included.  If those headers include <fcntl.h>, then
the #include in node_constants.cc will be a no-op and O_NONBLOCK won't
be visible.

/cc @indutny