build: Check whether rl_free_keymap is declared.

build: Check whether rl_free_keymap is declared.

Readline versions up to and including 6.2 miss to declare the public
symbol rl_free_keymap(). We currently work around this issue by
manually declaring the function in interactive.c.

The recently released readline-6.3, however, does declare that symbol.
Since we compile with -Wredundant-decls this leads to the following
warning:

	interactive.c:239:6: warning: redundant redeclaration of 'rl_free_keymap' [-Wredundant-decls]
	 void rl_free_keymap(Keymap keymap);

This patch gets rid of the warning by adding a check to configure.ac
which detects whether the symbol is declared in the readline header
file. Only if it is uncdelared we declare it manually as before.