Use _Atomic instead of volatile for thread safety where C11 is supported

Suggested by dodomorandi in #660

driver/level3/level3_gemm3m_thread.c

@@ -91,7 +91,12 @@
 #endif
 
 typedef struct {
-  volatile BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
+#if _STDC_VERSION__ >= 201112L  
+  _Atomic
+#else
+  volatile
+#endif  
+   BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
 } job_t;
 
 

driver/level3/level3_syrk_threaded.c

@@ -67,7 +67,12 @@
 #endif
 
 typedef struct {
-  volatile BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
+#if _STDC_VERSION__ >= 201112L  
+_Atomic
+#else 
+  volatile
+#endif
+   BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
 } job_t;
 
 

driver/level3/level3_thread.c

@@ -91,7 +91,12 @@
 #endif
 
 typedef struct {
-  volatile BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
+#if _STDC_VERSION__ >= 201112L
+_Atomic
+#else  
+  volatile
+#endif
+   BLASLONG working[MAX_CPU_NUMBER][CACHE_LINE_SIZE * DIVIDE_RATE];
 } job_t;