PGDLLIMPORT-ify MainLWLockArray, ProcDiePending, proc_exit_inprogress.

These are needed in HEAD to make assorted contrib modules build on Windows.
Now that all the MSVC and Mingw buildfarm members seem to be on the same
page about the need for them, we can have some confidence that future
problems of this ilk will be detected promptly; there seems nothing more
to be learned by delaying this fix further.

I chose to mark QueryCancelPending as well, since it's easy to imagine code
that wants to touch ProcDiePending also caring about QueryCancelPending.

src/include/miscadmin.h

@@ -74,8 +74,8 @@
 /* in globals.c */
 /* these are marked volatile because they are set by signal handlers: */
 extern PGDLLIMPORT volatile bool InterruptPending;
-extern volatile bool QueryCancelPending;
-extern volatile bool ProcDiePending;
+extern PGDLLIMPORT volatile bool QueryCancelPending;
+extern PGDLLIMPORT volatile bool ProcDiePending;
 
 extern volatile bool ClientConnectionLost;
 

src/include/storage/ipc.h

@@ -62,7 +62,7 @@ typedef void (*shmem_startup_hook_type) (void);
 
 
 /* ipc.c */
-extern bool proc_exit_inprogress;
+extern PGDLLIMPORT bool proc_exit_inprogress;
 
 extern void proc_exit(int code) __attribute__((noreturn));
 extern void shmem_exit(int code);

src/include/storage/lwlock.h

@@ -80,7 +80,7 @@ typedef union LWLockPadded
 	LWLock		lock;
 	char		pad[LWLOCK_PADDED_SIZE];
 } LWLockPadded;
-extern LWLockPadded *MainLWLockArray;
+extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
 
 /*
  * Some commonly-used locks have predefined positions within MainLWLockArray;