Move EAGAIN==EWOULDBLOCK check to config_static, rather than configure

I'm unaware of _any_ system which doesn't define these two as macros, so
placing it as a compile-time macro which would be portable across all
build systems would be nice.

Change-Id: Ia9ae0f2d9548594f29dbbbe87f42605b5b7445c5
Reviewed-on: http://review.couchbase.org/43535
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>