With ancient gcc, skip pg_attribute_printf() on function pointer.

Buildfarm results show that the ability to attach pg_attribute_printf
decoration to a function pointer appeared somewhere between gcc 2.95.3
and gcc 4.0.1.  Guess that it was there in 4.0.

src/bin/psql/psqlscan.h

@@ -32,7 +32,12 @@ typedef struct PsqlScanCallbacks
 	/* This pointer can be NULL if no variable substitution is wanted */
 	char	   *(*get_variable) (const char *varname, bool escape, bool as_ident);
 	/* Print an error message someplace appropriate */
+	/* (very old gcc versions don't support attributes on function pointers) */
+#if defined(__GNUC__) && __GNUC__ < 4
+	void		(*write_error) (const char *fmt,...);
+#else
 	void		(*write_error) (const char *fmt,...) pg_attribute_printf(1, 2);
+#endif
 } PsqlScanCallbacks;