Implement atomic_compare_exchange_strong in the atomic compat headers

This fixes building with MSVC (and older GCC versions) after
3e7886db.

include/compat/gcc/stdatomic.h

@@ -44,6 +44,7 @@ typedef unsigned int atomic_uint;
 #define atomic_fetch_sub(p_a, dec)    __atomic_fetch_sub(p_a, dec, __ATOMIC_SEQ_CST)
 #define atomic_exchange(p_a, v)       __atomic_exchange_n(p_a, v, __ATOMIC_SEQ_CST)
 #define atomic_fetch_or(p_a, v)       __atomic_fetch_or(p_a, v, __ATOMIC_SEQ_CST)
+#define atomic_compare_exchange_strong(p_a, expected, desired) __atomic_compare_exchange_n(p_a, expected, desired, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)
 
 #endif /* !defined(__cplusplus) */
 

include/compat/msvc/stdatomic.h

@@ -55,6 +55,15 @@ typedef enum {
 #define atomic_exchange(p_a, v)       InterlockedExchange(p_a, v)
 #define atomic_load_explicit(p_a, mo) atomic_load(p_a)
 
+static inline int atomic_compare_exchange_strong_int(LONG *obj, LONG *expected,
+                                                     LONG desired)
+{
+    LONG orig = *expected;
+    *expected = InterlockedCompareExchange(obj, desired, orig);
+    return *expected == orig;
+}
+#define atomic_compare_exchange_strong(p_a, expected, desired) atomic_compare_exchange_strong_int((LONG *)p_a, (LONG *)expected, (LONG)desired)
+
 /*
  * TODO use a special call to increment/decrement
  * using InterlockedIncrement/InterlockedDecrement