Without this fix, GCC 5.1 (correctly) warns about a subscript
error on the fts_name component of FTSENT. It's actually a
flexible member, so define it that way on C99 or later hosts.
* lib/fts.c (fts_alloc): Use offsetof, not sizeof, for a
structure that now has a flexible array member.
* lib/fts_.h (__FLEXIBLE_ARRAY_MEMBER): New macro.
(FTSENT): fts_name is now flexible on C99-or-later platforms.
* modules/fts (Depends-on): Add flexmember.