misc: remove unnecessary _GNU_SOURCE macros

Since we are building with the `-D_GNU_SOURCE` option turned on, the macro
definitions are not needed anymore.

Refs: https://github.com/libuv/libuv/pull/3165#issuecomment-835747442
PR-URL: https://github.com/libuv/libuv/pull/3168
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/unix/getaddrinfo.c

@@ -21,9 +21,6 @@
 /* Expose glibc-specific EAI_* error codes. Needs to be defined before we
  * include any headers.
  */
-#ifndef _GNU_SOURCE
-# define _GNU_SOURCE
-#endif
 
 #include "uv.h"
 #include "internal.h"

src/unix/linux-syscalls.h

@@ -22,9 +22,6 @@
 #ifndef UV_LINUX_SYSCALL_H_
 #define UV_LINUX_SYSCALL_H_
 
-#undef  _GNU_SOURCE
-#define _GNU_SOURCE
-
 #include <stdint.h>
 #include <signal.h>
 #include <sys/types.h>