Use a macro WUT_LIB_HEADER_{START,END}...

So we don't have to extern C and pragma pack in every header file.

include/coreinit/threadqueue.h

@@ -1,9 +1,7 @@
 #pragma once
 #include <wut.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+WUT_LIB_HEADER_START
 
 typedef struct OSThread OSThread;
 
@@ -48,6 +46,4 @@ void
 OSInitThreadQueueEx(OSThreadQueue *queue,
                     void *parent);
 
-#ifdef __cplusplus
-}
-#endif
+WUT_LIB_HEADER_END