* lib/dirname.h (DOUBLE_SLASH_IS_DISTINCT_ROOT): Default to 0, not 1.

* lib/dirname.h (DOUBLE_SLASH_IS_DISTINCT_ROOT): Default to 0, not 1.
This is more consistent with the other defines here.
* m4/double-slash-root.m4 (gl_DOUBLE_SLASH_ROOT):
Port to z/OS.  Problem reported by Paul Gilmartin.
Change local vars to use gl_ prefix rather than ac_.
Don't define DOUBLE_SLASH_IS_DISTINCT_ROOT to 0, for consistency
with other defines.
* modules/double-slash-root: New module.
* modules/dirname (Files): Remove m4/double-slash-root.m4.
(Depends-on): Add double-slash-root.
* MODULES.html.sh (File system functions): Mention new module.