PGDLLIMPORT'ify DateStyle and IntervalStyle.

This is needed on Windows to support contrib/postgres_fdw.  Although it's
been broken since last March, we didn't notice until recently because there
were no active buildfarm members that complained about missing PGDLLIMPORT
marking.  Efforts are underway to improve that situation, in support of
which we're delaying fixing some other cases of global variables that
should be marked PGDLLIMPORT.  However, this case affects 9.3, so we
can't wait any longer to fix it.

I chose to mark DateOrder as well, though it's not strictly necessary
for postgres_fdw.

src/include/miscadmin.h

@@ -202,8 +202,8 @@ extern PGDLLIMPORT Oid MyDatabaseTableSpace;
 #define DATEORDER_DMY			1
 #define DATEORDER_MDY			2
 
-extern int	DateStyle;
-extern int	DateOrder;
+extern PGDLLIMPORT int DateStyle;
+extern PGDLLIMPORT int DateOrder;
 
 /*
  * IntervalStyles
@@ -217,7 +217,7 @@ extern int	DateOrder;
 #define INTSTYLE_SQL_STANDARD		2
 #define INTSTYLE_ISO_8601			3
 
-extern int	IntervalStyle;
+extern PGDLLIMPORT int IntervalStyle;
 
 #define MAXTZLEN		10		/* max TZ name len, not counting tr. null */