Use a typedef to declare extern atomic variables

Some systems use anonymous structs for atomic storage, and extern declarations
need to have the same type as their non-extern definition.

router/al.c

@@ -7,7 +7,7 @@
 #include "router.h"
 
 
-ATOMIC(DriverIface*) CurrentCtxDriver = ATOMIC_INIT_STATIC(NULL);
+atomic_DriverIfacePtr CurrentCtxDriver = ATOMIC_INIT_STATIC(NULL);
 
 #define DECL_THUNK1(R,n,T1) AL_API R AL_APIENTRY n(T1 a)                      \
 {                                                                             \

router/router.h

@@ -137,7 +137,8 @@ extern DriverIface *DriverList;
 extern int DriverListSize;
 
 extern altss_t ThreadCtxDriver;
-extern ATOMIC(DriverIface*) CurrentCtxDriver;
+typedef ATOMIC(DriverIface*) atomic_DriverIfacePtr;
+extern atomic_DriverIfacePtr CurrentCtxDriver;
 
 
 typedef struct PtrIntMap {