fix -Wunused-but-set-variable for gcc-4.6 on x64

src/platform-linux.cc

Index: src/platform-linux.cc
diff --git a/src/platform-linux.cc b/src/platform-linux.cc
index ab22a79cd4294a24c310308e8a9ef4fec2413a91..ea8e5f014201b9a8b89fbfdb76574ab2226e3c73 100644
--- a/src/platform-linux.cc
+++ b/src/platform-linux.cc
@@ -728,12 +728,15 @@ class LinuxMutex : public Mutex {
  public:
   LinuxMutex() {
     pthread_mutexattr_t attrs;
-    int result = pthread_mutexattr_init(&attrs);
-    ASSERT(result == 0);
-    result = pthread_mutexattr_settype(&attrs, PTHREAD_MUTEX_RECURSIVE);
-    ASSERT(result == 0);
-    result = pthread_mutex_init(&mutex_, &attrs);
-    ASSERT(result == 0);
+#ifdef DEBUG
+#define CHECK_RESULT(exp) ASSERT((exp) == 0)
+#else
+#define CHECK_RESULT(exp) exp
+#endif
+    CHECK_RESULT(pthread_mutexattr_init(&attrs));
+    CHECK_RESULT(pthread_mutexattr_settype(&attrs, PTHREAD_MUTEX_RECURSIVE));
+    CHECK_RESULT(pthread_mutex_init(&mutex_, &attrs));
+#undef CHECK_RESULT
   }
 
   virtual ~LinuxMutex() { pthread_mutex_destroy(&mutex_); }