Support building without greenlet

greenlet has some overhead.
I'd like to have option for benchmark contest.

test code:

```python
def app(env, start):
    start("200 OK", [('Content-Type', 'text/plain')])
    return [b'Hello, World\n']
```

With greenlet:

```
$ wrk http://localhost:6060/
Running 10s test @ http://localhost:6060/
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.31ms  426.45us   9.21ms   92.52%
    Req/Sec     4.12k   679.67     5.55k    71.28%
  77864 requests in 10.00s, 13.44MB read
Requests/sec:   7786.27
Transfer/sec:      1.34MB
```

Without greenlet:

```
$ wrk http://localhost:6060/
Running 10s test @ http://localhost:6060/
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   458.80us  180.41us   3.80ms   78.34%
    Req/Sec    11.23k     1.59k   17.78k    72.08%
  212357 requests in 10.00s, 36.66MB read
Requests/sec:  21236.50
Transfer/sec:      3.67MB
```