Ensure that <features.h> is included before __GLIBC__ is tested.

Ensure that <features.h> is included before __GLIBC__ is tested.

* lib/printf-parse.h: Include <features.h>.
* m4/vasnprintf.m4 (gl_PREREQ_PRINTF_PARSE): Require gl_FEATURES_H.

* lib/wchar.in.h: Include <features.h>.
* m4/wchar_h.m4 (gl_WCHAR_H): Require gl_FEATURES_H.
* modules/wchar (Makefile.am): Substitute HAVE_FEATURES_H.

* lib/arpa_inet.in.h: Include <features.h>.
* m4/arpa_inet_h.m4 (gl_HEADER_ARPA_INET): Require gl_FEATURES_H.
* modules/arpa_inet (Makefile.am): Substitute HAVE_FEATURES_H.

* build-aux/link-warning.h: Include <features.h>.
* modules/link-warning (configure.ac): Require gl_FEATURES_H.
(Makefile.am): Substitute HAVE_FEATURES_H into link-warning.h.

* m4/gnulib-common.m4 (gl_FEATURES_H): New macro.
Reported by Mike Frysinger <vapier@gentoo.org>.