Do not define HAVE_CLOCK_GETTIME_MONOTONIC on macOS

`clock_gettime` wasn't added until macOS 10.12, which causes `dyld: Symbol not found: _clock_gettime` error

Author: ohadravid

curl-sys/build.rs

@@ -271,10 +271,11 @@ fn main() {
     } else {
         if target.contains("-apple-") {
             cfg.define("__APPLE__", None).define("macintosh", None);
+        } else {
+            cfg.define("HAVE_CLOCK_GETTIME_MONOTONIC", None);
         }
 
         cfg.define("RECV_TYPE_ARG1", "int")
-            .define("HAVE_CLOCK_GETTIME_MONOTONIC", None)
             .define("HAVE_GETTIMEOFDAY", None)
             .define("HAVE_PTHREAD_H", None)
             .define("HAVE_ARPA_INET_H", None)