Use C99-style flexible array members if available.

Use C99-style flexible array members if available.

This avoids some subtle aliasing issues, which typically
aren't a problem with GCC but may be a problem elsewhere.
* lib-src/ebrowse.c (struct member, struct alias, struct sym):
Use FLEXIBLE_ARRAY_MEMBER.
(add_sym, add_member, make_namespace, register_namespace_alias):
Use offsetof (struct, flex_array_member), not sizeof (struct), as
that ports better to pre-C99 non-GCC.
* src/alloc.c (sdata): New typedef, replacing the old struct sdata.
It is a struct if GC_CHECK_STRING_BYTES, a union otherwise.
In either case, it uses a flexible array member rather than
the old struct hack.  All uses changed.
(SDATA_NBYTES, sweep_strings) [!GC_CHECK_STRING_BYTES]:
Adjust to sdata reorganization.
* src/alloc.c (VBLOCK_BYTES_MIN, allocate_vectorlike, Fgarbage_collect):
Use offsetof (struct, flex_array_member), not sizeof (struct), as
that ports better to pre-C99 non-GCC.
* src/chartab.c (Fmake_char_table, make_sub_char_table, copy_char_table):
Use CHAR_TABLE_STANDARD_SLOTS rather than its definition,
as the latter has changed.
* src/conf_post.h (FLEXIBLE_ARRAY_MEMBER): Move here from w32.c,
and port better to pre-C99 GCC.
* src/image.c (struct xpm_cached_color):
* src/lisp.h (struct Lisp_Vector, struct Lisp_Bool_Vector)
(struct Lisp_Char_Table, struct Lisp_Sub_Char_Table):
Use FLEXIBLE_ARRAY_MEMBER.
* src/lisp.h (string_bytes) [GC_CHECK_STRING_BYTES]:
Move decl to top level so it gets checked against implementation.
(CHAR_TABLE_STANDARD_SLOTS): Adjust to struct Lisp_Char_Table change.
* src/w32.c (FLEXIBLE_ARRAY_MEMBER): Move to conf_post.h.