Avoid clobbering potential defines that were set up externally.

Change on 2015/01/12 by nnoble <nnoble@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=83799744

src/core/support/cpu_linux.c

@@ -37,13 +37,37 @@
 
 #include "src/core/support/cpu.h"
 
+#ifndef _GNU_SOURCE
 #define _GNU_SOURCE
+#define GRPC_GNU_SOURCE
+#endif
+
+#ifndef __USE_GNU
 #define __USE_GNU
+#define GRPC_USE_GNU
+#endif
+
+#ifndef __USE_MISC
 #define __USE_MISC
+#define GRPC_USE_MISC
+#endif
+
 #include <sched.h>
+
+#ifdef GRPC_GNU_SOURCE
 #undef _GNU_SOURCE
+#undef GRPC_GNU_SOURCE
+#endif
+
+#ifdef GRPC_USE_GNU
 #undef __USE_GNU
+#undef GRPC_USE_GNU
+#endif
+
+#ifdef GRPC_USE_MISC
 #undef __USE_MISC
+#undef GRPC_USE_MISC
+#endif
 
 #include <errno.h>
 #include <unistd.h>