Add 'getthrds' declaration check in configure for AIX

Add 'getthrds' declaration check in configure for AIX

On newer versions of AIX (6.x and later), this function is actually
declared in procinfo.h, thus causing a compilation warning when we
re-declare it ourselves. This patch adds a configure check for that
function allowing us to declare the function only if the declaration
isn't already present in one of procinfo system header.

gdb/ChangeLog:

        PR gdb/12116:
        * configure.ac: Add getthrds declaration check.
        * configure, config.in: Regenerate.
        * aix-thread.c (getthrds): Declare only if not already declared
        in procinfo.h.  More declaration out of get_signaled_thread to
        global scope.