acl, mbchar, priv-set: use extern-inline

acl, mbchar, priv-set: use extern-inline

* lib/set-mode-acl.c, lib/acl-internal.h (ACL_INTERNAL_INLINE):
* lib/mbchar.c, lib/mbchar.h (MBCHAR_INLINE):
* lib/priv-set.c, lib/priv-set.h (PRIV_SET_INLINE):
New macros.
* lib/acl-internal.h, lib/mbchar.h, lib/priv-set.h:
Replace all uses of 'static inline' with it.
Use _GL_INLINE_HEADER_BEGIN, _GL_INLINE_HEADER_END.
* m4/acl.m4 (gl_FUNC_ACL):
* m4/mbchar.m4 (gl_MBCHAR):
* m4/priv-set.m4 (gl_PRIV_SET):
Remove AC_C_INLINE, since 'inline' is no longer used directly.
* modules/acl, modules/mbchar, modules/priv-set (Depends-on):
Add extern-inline.