Check for gethrtime. Needed for solais/joyent

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