Use expression statements also on clang.

Use expression statements also on clang.

* lib/cdefs.h (__extension__): Don't define to empty on clang.
* lib/obstack.h (__extension__): Likewise.
(obstack_object_size, obstack_room, obstack_make_room, obstack_empty_p,
obstack_grow, obstack_grow0, obstack_1grow, obstack_ptr_grow,
obstack_int_grow, obstack_ptr_grow_fast, obstack_int_grow_fast,
obstack_blank, obstack_alloc, obstack_copy, obstack_copy0,
obstack_finish, obstack_free): Enable as optimized macros also on clang.
* lib/fpucw.h (fpucw_t etc.): Enable the definitions also on clang.