Check for gethrtime. Needed for solaris/smartos

The actual guard condition in the header already[1], this patch just
make it defined on the systems where gethrtime() defined, and disables
internal implementation

[1]: https://github.com/couchbase/couchbase-ruby-client/blob/master/ext/couchbase_ext/couchbase_ext.h#L32-L35

Change-Id: Ie4d1518e0ec41e30ecc849d93b26322c80d06de2
Reviewed-on: http://review.couchbase.org/24537
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>