Add configure probe for rl_variable_bind().

Some exceedingly ancient readline libraries lack this function, causing
commit 3d858af07 to fail.  Per buildfarm (via Michael Paquier).

Discussion: https://postgr.es/m/E1msTLm-0007Cm-Ri@gemulon.postgresql.org

src/bin/psql/input.c

@@ -356,8 +356,10 @@ initializeInput(int flags)
 		/* set appropriate values for Readline's global variables */
 		initialize_readline();
 
+#ifdef HAVE_RL_VARIABLE_BIND
 		/* set comment-begin to a useful value for SQL */
 		(void) rl_variable_bind("comment-begin", "-- ");
+#endif
 
 		/* this reads ~/.inputrc, so do it after rl_variable_bind */
 		rl_initialize();

src/include/pg_config.h.in

@@ -485,6 +485,9 @@
 /* Define to 1 if you have the `rl_reset_screen_size' function. */
 #undef HAVE_RL_RESET_SCREEN_SIZE
 
+/* Define to 1 if you have the `rl_variable_bind' function. */
+#undef HAVE_RL_VARIABLE_BIND
+
 /* Define to 1 if you have the <security/pam_appl.h> header file. */
 #undef HAVE_SECURITY_PAM_APPL_H
 

src/tools/msvc/Solution.pm

@@ -354,6 +354,7 @@ sub GenerateFiles
 		HAVE_RL_FILENAME_QUOTE_CHARACTERS        => undef,
 		HAVE_RL_FILENAME_QUOTING_FUNCTION        => undef,
 		HAVE_RL_RESET_SCREEN_SIZE                => undef,
+		HAVE_RL_VARIABLE_BIND                    => undef,
 		HAVE_SECURITY_PAM_APPL_H                 => undef,
 		HAVE_SETENV                              => undef,
 		HAVE_SETPROCTITLE                        => undef,