Use stdbool.h if suitable

Using the standard bool type provided by C allows some recent compilers
and debuggers to give better diagnostics.  Also, some extension code and
third-party headers are increasingly pulling in stdbool.h, so it's
probably saner if everyone uses the same definition.

But PostgreSQL code is not prepared to handle bool of a size other than
1, so we keep our own old definition if we encounter a stdbool.h with a
bool of a different size.  (Among current build farm members, this only
applies to old macOS versions on PowerPC.)

To check that the used bool is of the right size, add a static
assertions about size of GinTernaryValue vs bool.  This is currently the
only place that assumes that bool and char are of the same size.

Discussion: https://www.postgresql.org/message-id/flat/3a0fe7e1-5ed1-414b-9230-53bbc0ed1f49@2ndquadrant.com

src/backend/utils/adt/tsginidx.c

@@ -309,7 +309,9 @@ gin_tsquery_consistent(PG_FUNCTION_ARGS)
 		 * query.
 		 */
 		gcv.first_item = GETQUERY(query);
-		gcv.check = check;
+		StaticAssertStmt(sizeof(GinTernaryValue) == sizeof(bool),
+						 "sizes of GinTernaryValue and bool are not equal");
+		gcv.check = (GinTernaryValue *) check;
 		gcv.map_item_operand = (int *) (extra_data[0]);
 		gcv.need_recheck = recheck;