Add GUC variables for stat tracking and timeout as PGDLLIMPORT

This helps integration of extensions with Windows.  The following
parameters are changed:
- idle_in_transaction_session_timeout (9.6 and newer versions)
- lock_timeout
- statement_timeout
- track_activities
- track_counts
- track_functions

Author: Pascal Legrand
Reviewed-by: Amit Kamila, Julien Rouhaud, Michael Paquier
Discussion: https://postgr.es/m/1579298868581-0.post@n3.nabble.com
Backpatch-through: 9.4

src/include/pgstat.h

@@ -1210,9 +1210,9 @@ typedef struct PgStat_FunctionCallUsage
  * GUC parameters
  * ----------
  */
-extern bool pgstat_track_activities;
-extern bool pgstat_track_counts;
-extern int	pgstat_track_functions;
+extern PGDLLIMPORT bool pgstat_track_activities;
+extern PGDLLIMPORT bool pgstat_track_counts;
+extern PGDLLIMPORT int pgstat_track_functions;
 extern PGDLLIMPORT int pgstat_track_activity_query_size;
 extern char *pgstat_stat_directory;
 extern char *pgstat_stat_tmpname;

src/include/storage/proc.h

@@ -293,9 +293,9 @@ extern PGPROC *PreparedXactProcs;
 
 /* configurable options */
 extern PGDLLIMPORT int DeadlockTimeout;
-extern int	StatementTimeout;
-extern int	LockTimeout;
-extern int	IdleInTransactionSessionTimeout;
+extern PGDLLIMPORT int StatementTimeout;
+extern PGDLLIMPORT int LockTimeout;
+extern PGDLLIMPORT int IdleInTransactionSessionTimeout;
 extern bool log_lock_waits;