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-shared.h

@@ -28,10 +28,10 @@
 #include <stdlib.h>
 #include <stdbool.h>
 #include <assert.h>
-#include <stdatomic.h>
 #include <stdint.h>
 
 #include "m-core.h"
+#include "m-atomic.h"
 
 /* Define the oplist of a shared pointer.
    USAGE: SHARED_OPLIST(name [, oplist_of_the_type]) */