Use FLEXIBLE_ARRAY_MEMBER in a bunch more places.

Replace some bogus "x[1]" declarations with "x[FLEXIBLE_ARRAY_MEMBER]".
Aside from being more self-documenting, this should help prevent bogus
warnings from static code analyzers and perhaps compiler misoptimizations.

This patch is just a down payment on eliminating the whole problem, but
it gets rid of a lot of easy-to-fix cases.

Note that the main problem with doing this is that one must no longer rely
on computing sizeof(the containing struct), since the result would be
compiler-dependent.  Instead use offsetof(struct, lastfield).  Autoconf
also warns against spelling that offsetof(struct, lastfield[0]).

Michael Paquier, review and additional fixes by me.

src/include/access/gist_private.h

@@ -47,7 +47,7 @@ typedef struct
 {
 	BlockNumber prev;
 	uint32		freespace;
-	char		tupledata[1];
+	char		tupledata[FLEXIBLE_ARRAY_MEMBER];
 } GISTNodeBufferPage;
 
 #define BUFFER_PAGE_DATA_OFFSET MAXALIGN(offsetof(GISTNodeBufferPage, tupledata))
@@ -131,7 +131,8 @@ typedef struct GISTSearchItem
 		/* we must store parentlsn to detect whether a split occurred */
 		GISTSearchHeapItem heap;	/* heap info, if heap tuple */
 	}			data;
-	double		distances[1];	/* array with numberOfOrderBys entries */
+	double		distances[FLEXIBLE_ARRAY_MEMBER];		/* numberOfOrderBys
+														 * entries */
 } GISTSearchItem;
 
 #define GISTSearchItemIsHeap(item)	((item).blkno == InvalidBlockNumber)
@@ -144,7 +145,7 @@ typedef struct GISTSearchItem
 typedef struct GISTScanOpaqueData
 {
 	GISTSTATE  *giststate;		/* index information, see above */
-	pairingheap *queue;		/* queue of unvisited items */
+	pairingheap *queue;			/* queue of unvisited items */
 	MemoryContext queueCxt;		/* context holding the queue */
 	bool		qual_ok;		/* false if qual can never be satisfied */
 	bool		firstCall;		/* true until first gistgettuple call */