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

This change is picked from https://github.com/libevent/libevent/commit/ea6b1df

BUG=455089

Review URL: https://codereview.chromium.org/895323002

Cr-Commit-Position: refs/heads/master@{#314761}

third_party/libevent/README.chromium

@@ -23,3 +23,4 @@ static library using GYP.
    nacl_nonsfi/event-config.h are derived from linux/ counterparts.
    nacl_nonsfi/random.c is also added to provide the random() function,
    which is missing in the newlib-based PNaCl toolchain.
+8) Apply https://github.com/libevent/libevent/commit/ea6b1df

third_party/libevent/evdns.c

@@ -55,7 +55,9 @@
 #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