Restore _Atomic define before stdatomic.h for old gcc

see #1593

Author: martin-frbg

common.h

@@ -649,12 +649,21 @@ int omp_get_num_procs(void);
 __declspec(dllimport) int __cdecl omp_in_parallel(void);
 __declspec(dllimport) int __cdecl omp_get_num_procs(void);
 #endif
+
 #if (__STDC_VERSION__ >= 201112L)
+#if defined(C_GCC) && ( __GNUC__ < 7) 
+// workaround for GCC bug 65467
 #ifndef _Atomic
 #define _Atomic volatile
 #endif
+#endif
 #include <stdatomic.h>
+#else
+#ifndef _Atomic
+#define _Atomic volatile
 #endif
+
+
 #else
 #ifdef __ELF__
 int omp_in_parallel  (void) __attribute__ ((weak));