This works better than 'assert' when compiling with -DNDEBUG,
as it avoids some compiler diagnostics in that case.
Reported by Norihiro Tanaka in:
http://lists.gnu.org/archive/html/bug-gnulib/2014-12/msg00215.html
* MODULES.html.sh (func_all_modules): Add 'assure'.
* lib/assure.h, modules/assure: New files.
* lib/chdir-long.c, lib/cycle-check.c, lib/fchdir.c, lib/fts.c:
* lib/poll.c, lib/savewd.c, lib/utimens.c, lib/xstrtol.c:
Prefer 'assure' to 'assert'.
* modules/chdir-long, modules/cycle-check, modules/fchdir:
* modules/poll, modules/savewd, modules/utimens, modules/xstrtol:
Depend on 'assure'.