when built against readline 4.2.  Specifically, it handles the deprecation
of

    filename_completion_function()

with preference for

    rl_filename_completion_function()

Although, I was motivated by Cygwin support, IMO this patch is appropriate
for all platforms.  To quote from the readline source:

    #if 0
    /* Backwards compatibility (compat.c).  These will go away sometime. */
    ...
    extern READLINE_EXPORT(char, *filename_completion_function) ...
    #endif

Note that this patch is modeled after the one by Peter Eisentraut for
completion_matches():

    http://www.ca.postgresql.org/~petere/readline42.html
I tested this patch under the following environments:

    Cygwin with readline 4.1
    Cygwin with readline 4.2
    Linux with readline 2.2.1
    Linux with readline 4.2

and it behaved as expected.

Jason Tishler