libevent: Only define _GNU_SOURCE if it is not already defined.

third_party/libevent/evdns.c

 /* $Id: evdns.c 6979 2006-08-04 18:31:13Z nickm $ */
 
 /* The original version of this module was written by Adam Langley; for
  * a history of modifications, check out the subversion logs.
  *
  * When editing this module, try to keep it re-mergeable by Adam.  Don't
  * reformat the whitespace, add Tor dependencies, or so on.
  *
  * TODO:
  *   - Support IPv6 and PTR records.
@@ skipping 37 matching lines @@
 #ifndef DNS_USE_OPENSSL_FOR_ID
 #ifndef DNS_USE_FTIME_FOR_ID
 #error Must configure at least one id generation method.
 #error Please see the documentation.
 #endif
 #endif
 #endif
 #endif
 
 /* #define _POSIX_C_SOURCE 200507 */
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
+#endif
 
 #ifdef DNS_USE_CPU_CLOCK_FOR_ID
 #ifdef DNS_USE_OPENSSL_FOR_ID
 #error Multiple id options selected
 #endif
 #ifdef DNS_USE_GETTIMEOFDAY_FOR_ID
 #error Multiple id options selected
 #endif
 #include <time.h>
 #endif
@@ skipping 3110 matching lines @@
                 } else {
                         fprintf(stderr, "resolving (fwd) %s...\n",v[idx]);
                         evdns_resolve_ipv4(v[idx], 0, main_callback, v[idx]);
                 }
         }
         fflush(stdout);
         event_dispatch();
         return 0;
 }
 #endif