Fix up feature test macros
Move all feature test macros to the start of the file and check that
they aren't already defined or defined to a lower value than the file
needs.
Projects should be allowed to put these in CFLAGS and we shouldn't break
when they do.
diff --git a/src/core/iomgr/tcp_server_posix.c b/src/core/iomgr/tcp_server_posix.c
index d169d23..091f0aa 100644
--- a/src/core/iomgr/tcp_server_posix.c
+++ b/src/core/iomgr/tcp_server_posix.c
@@ -31,11 +31,15 @@
*
*/
+/* FIXME: "posix" files shouldn't be depending on _GNU_SOURCE */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
#include <grpc/support/port_platform.h>
#ifdef GPR_POSIX_SOCKET
-#define _GNU_SOURCE
#include "src/core/iomgr/tcp_server.h"
#include <limits.h>