Improve sys_siglist detection.

Improve sys_siglist detection.

* configure.ac (sys_siglist): Look for its decl in <signal.h>.
Otherwise, it's not found in either Fedora 17 or Solaris 11.
* src/sysdep.c (sys_siglist, init_signals): Use _sys_siglist if it's
defined as a macro, as is done in Solaris.
(sys_siglist_entries): New macro.
(save_strsignal): Use it.
* src/syssignal.h (safe_strsignal): Now ATTRIBUTE_CONST, to pacify
GCC 4.7.2 on Fedora 17 with the fixed sys_siglist detection.