[v3] system_error

[Benjamin Kosnik]

> include/system_error:100: error: 'ENODATA' was not declared in this
> include/system_error:104: error: 'ENOSR' was not declared in this
> include/system_error:111: error: 'ENOSTR' was not declared in this
> include/system_error:135: error: 'ETIME' was not declared in this

ok. well, I suppose these could be checked just like the other macros
were checked for netbsd.

this won't help with the cygwin case though. And I suppose we'll have
to come up with a substitute for those platforms that don't implement
the necessary bits.

-benjamin