Replace stdatomic.h (C11 standard compliant header) by a custom one

to increase C++ compatibility as stdatomic.h doesn't work in C++!

(I would like to thanks the one(s) who thought it was a good idea
***not*** to include stdatomic.h -and thread.h- in the C++ standard!)

m-snapshot.h

@@ -26,8 +26,8 @@
 #define __M_SNAPSHOT
 
 #include <assert.h>
-#include <stdatomic.h>
 
+#include "m-atomic.h"
 #include "m-core.h"
 
 /* Define a snapshot and it function