Make poll() the default (#2065)

* Make poll() the default

select() can still be enabled by defining CPPHTTPLIB_USE_SELECT.

* Run tests with select() and poll()

README.md

@@ -872,10 +872,10 @@ res->body; // Compressed data
 
 ```
 
-Use `poll` instead of `select`
-------------------------------
+Use `select()` instead of `poll()`
+----------------------------------
 
-`select` system call is used as default since it's more widely supported. If you want to let cpp-httplib use `poll` instead, you can do so with `CPPHTTPLIB_USE_POLL`.
+cpp-httplib defaults to the widely supported `poll()` system call. If your OS lacks support for `poll()`, define `CPPHTTPLIB_USE_SELECT` to use `select()` instead.
 
 Unix Domain Socket Support
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