Disable compiling with poll() on macOS (#393)

I've noticed recently that when using the bundled curl version on macOS, very occasionally curl will get "stuck" in the TCP connection phase when using the multi interface. Apparently this is caused by the `poll()` implementation not behaving quite as the POSIX standard says it should on some macOS versions.

This originally was a problem just for macOS 10.12, but after a recent update it seems like this regression might have come back in 10.15. It is better to just disable `poll()` completely on macOS, which the official build configuration does here: https://github.com/curl/curl/blob/1e19eceb50d6d8b9c9b37e2f8e1259b2f311c82a/m4/curl-functions.m4#L4999-L5010

See also curl/curl#1057 when this was originally reported way back in the day.

curl-sys/build.rs

@@ -300,7 +300,6 @@ fn main() {
             .define("HAVE_NETDB_H", None)
             .define("HAVE_NETINET_IN_H", None)
             .define("HAVE_NETINET_TCP_H", None)
-            .define("HAVE_POLL_FINE", None)
             .define("HAVE_POLL_H", None)
             .define("HAVE_FCNTL_O_NONBLOCK", None)
             .define("HAVE_SYS_SELECT_H", None)
@@ -336,7 +335,11 @@ fn main() {
                 .define("HAVE_MACH_ABSOLUTE_TIME", None);
         } else {
             cfg.define("HAVE_CLOCK_GETTIME_MONOTONIC", None)
-                .define("HAVE_GETTIMEOFDAY", None);
+                .define("HAVE_GETTIMEOFDAY", None)
+                // poll() on various versions of macOS are janky, so only use it
+                // on non-macOS unix-likes. This matches the official default
+                // build configuration as well.
+                .define("HAVE_POLL_FINE", None);
         }
 
         if cfg!(feature = "spnego") {