Add macros wrapping all usage of gcc's __attribute__.

Until now __attribute__() was defined to be empty for all compilers but
gcc. That's problematic because it prevents using it in other compilers;
which is necessary e.g. for atomics portability.  It's also just
generally dubious to do so in a header as widely included as c.h.

Instead add pg_attribute_format_arg, pg_attribute_printf,
pg_attribute_noreturn macros which are implemented in the compilers that
understand them. Also add pg_attribute_noreturn and pg_attribute_packed,
but don't provide fallbacks, since they can affect functionality.

This means that external code that, possibly unwittingly, relied on
__attribute__ defined to be empty on !gcc compilers may now run into
warnings or errors on those compilers. But there shouldn't be many
occurances of that and it's hard to work around...

Discussion: 54B58BA3.8040302@ohmu.fi
Author: Oskari Saarenmaa, with some minor changes by me.

contrib/cube/cubescan.l

@@ -60,7 +60,7 @@ float        ({integer}|{real})([eE]{integer})?
 
 %%
 
-void __attribute__((noreturn))
+void pg_attribute_noreturn
 yyerror(NDBOX **result, const char *message)
 {
 	if (*yytext == YY_END_OF_BUFFER_CHAR)

contrib/pg_upgrade/pg_upgrade.h

@@ -359,7 +359,7 @@ void		optionally_create_toast_tables(void);
 bool
 exec_prog(const char *log_file, const char *opt_log_file,
 		  bool throw_error, const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 4, 5)));
+pg_attribute_printf(4, 5);
 void		verify_directories(void);
 bool		pid_lock_file_exists(const char *datadir);
 
@@ -445,7 +445,7 @@ void		init_tablespaces(void);
 PGconn	   *connectToServer(ClusterInfo *cluster, const char *db_name);
 PGresult *
 executeQueryOrDie(PGconn *conn, const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 3)));
+pg_attribute_printf(2, 3);
 
 char	   *cluster_conn_opts(ClusterInfo *cluster);
 
@@ -462,17 +462,17 @@ int			get_user_info(char **user_name_p);
 void		check_ok(void);
 void
 report_status(eLogType type, const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 3)));
+pg_attribute_printf(2, 3);
 void
 pg_log(eLogType type, const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 3)));
+pg_attribute_printf(2, 3);
 void
 pg_fatal(const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 1, 2), noreturn));
+pg_attribute_printf(1, 2) pg_attribute_noreturn;
 void		end_progress_output(void);
 void
 prep_status(const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 1, 2)));
+pg_attribute_printf(1, 2);
 void		check_ok(void);
 const char *getErrorText(int errNum);
 unsigned int str2uint(const char *str);
@@ -489,7 +489,7 @@ void old_9_3_check_for_line_data_type_usage(ClusterInfo *cluster);
 void
 parallel_exec_prog(const char *log_file, const char *opt_log_file,
 				   const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 3, 4)));
+pg_attribute_printf(3, 4);
 void parallel_transfer_all_new_dbs(DbInfoArr *old_db_arr, DbInfoArr *new_db_arr,
 							  char *old_pgdata, char *new_pgdata,
 							  char *old_tablespace);

contrib/pg_upgrade/util.c

@@ -82,7 +82,7 @@ prep_status(const char *fmt,...)
 
 
 static
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 0)))
+pg_attribute_printf(2, 0)
 void
 pg_log_v(eLogType type, const char *fmt, va_list ap)
 {

contrib/pg_xlogdump/pg_xlogdump.c

@@ -69,7 +69,7 @@ typedef struct XLogDumpStats
 
 static void
 fatal_error(const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 1, 2)));
+pg_attribute_printf(1, 2);
 
 /*
  * Big red button to push when things go horribly wrong.

contrib/pgcrypto/px.h

@@ -208,7 +208,7 @@ void		px_memset(void *ptr, int c, size_t len);
 #ifdef PX_DEBUG
 void
 px_debug(const char *fmt,...)
-__attribute__((format(PG_PRINTF_ATTRIBUTE, 1, 2)));
+pg_attribute_printf(1, 2);
 #else
 #define px_debug(...)
 #endif

contrib/seg/segscan.l

@@ -59,7 +59,7 @@ float        ({integer}|{real})([eE]{integer})?
 
 %%
 
-void __attribute__((noreturn))
+void pg_attribute_noreturn
 yyerror(SEG *result, const char *message)
 {
 	if (*yytext == YY_END_OF_BUFFER_CHAR)