On Fedora 20, I got this compiler warning:

/home/sky/Code/python/cpython3.4/Modules/readline.c: In function ‘flex_complete’:
/home/sky/Code/python/cpython3.4/Modules/readline.c:962:5: warning: passing argument 1 of ‘completion_matches’ discards ‘const’ qualifier from pointer target type [enabled by default]
     result = completion_matches(text, *on_completion);
     ^
/home/sky/Code/python/cpython3.4/Modules/readline.c:40:15: note: expected ‘char *’ but argument is of type ‘const char *’
 extern char **completion_matches(char *, rl_compentry_func_t *);
               ^

Here is the patch using Christian Heimes' strategy in silencing compiler warning in dbm module.
http://hg.python.org/cpython/rev/3068ff3d9d1e

What will happen if change the signature of completion_matches to "const char*"?

It works. I chose Christian Heimes' strategy because of: 

char ** completion_matches (char *text, CPFunction *entry_func)
in
http://web.mit.edu/gnu/doc/html/rlman_2.html

Interesting, how compiler got this warning? In readline6 the declarations of legacy functions including completion_matches are in "#if 0" block. In libedit it declared with "const char *".

It's weird. When I make clean and configure again and make again, the error warning message disappear. Prior to that, I could confirm, I was in this block:

#ifdef HAVE_RL_COMPLETION_MATCHES
#define completion_matches(x, y) \
    rl_completion_matches((x), ((rl_compentry_func_t *)(y)))
#else
#if defined(_RL_FUNCTION_TYPEDEF)

Here I am!!!!!!!!!!!!!!!!!!!!

extern char **completion_matches(char *, rl_compentry_func_t *);
#else

#if !defined(__APPLE__)
extern char **completion_matches(char *, CPFunction *);
#endif
#endif
#endif

Then perhaps this is just misconfiguration.