Make atomics compatible in more compiler modes

The typedef case is questionable; still thinking if it should error
there. On one hand, the effect on the code is no different than what
is used today, however, it may have the effect on programmers that it
is actually atomic, which could be bad.

Author: morrisonlevi

Zend/zend_globals.h

@@ -21,7 +21,6 @@
 #define ZEND_GLOBALS_H
 
 #include <setjmp.h>
-#include <stdatomic.h>
 
 #include "zend_globals_macros.h"
 
@@ -37,6 +36,21 @@
 #include "zend_multiply.h"
 #include "zend_arena.h"
 
+/* Unlike stdbool.h, not all supported compilers support stdatomic.h in C++
+ * mode. So, conditionally include <atomic> or <stdatomic.h> depending on
+ * whether the compiler is in C++ mode or not. This restricts us to functions,
+ * types, and macros which are present in both, which is a drag.
+ */
+#ifdef __cplusplus
+#include <atomic>
+using std::atomic_bool;
+#elif !defined(__STDC_NO_ATOMICS__)
+#include <stdatomic.h>
+#else
+// fall back to non-atomics
+typedef bool atomic_bool
+#endif
+
 /* Define ZTS if you want a thread-safe Zend */
 /*#undef ZTS*/