Unlike most other autoconf defines, this is *defined to 0* when the check symbol is not found. (See info autoconf, search on "Macro: AC_CHECK_DECLS")
The usage in threadproc/unix/signals.c: #if defined(SYS_SIGLIST_DECLARED) || defined(HAVE_DECL_SYS_SIGLIST) is therefore incorrect. Instead, it should be: #if defined(SYS_SIGLIST_DECLARED) || HAVE_DECL_SYS_SIGLIST Max.
