Problems reported by Gisle Vanem in:
http://lists.gnu.org/archive/html/bug-gnulib/2017-09/msg00016.html
* lib/libc-config.h (__inline): Don't define if HAVE___INLINE.
(libc_hidden_proto): Stick to Standard C syntax for varargs macro.
* m4/__inline.m4: New file.
* modules/libc-config (Files): Add it.
(Depends-on): Use it.
