link-warning: make usage consistent

link-warning: make usage consistent

Ensure GL_LINK_WARNING is defined before use.

* modules/ctype (Depends-on): Add link-warning.
(Makefile.am): Update rules accordingly.
* modules/langinfo (Depends-on, Makefile.am): Likewise.
* modules/locale (Depends-on, Makefile.am): Likewise.
* modules/sys_file (Makefile.am): Likewise.
* modules/getopt-posix (Makefile.am): Delete unused link warning
efforts.
* lib/ctype.in.h (GL_LINK_WARNING): Ensure definition before use.
* lib/langinfo.in.h (GL_LINK_WARNING): Likewise.
* lib/locale.in.h (GL_LINK_WARNING): Likewise.
* lib/sys_file.in.h (GL_LINK_WARNING): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>