locale: Provide a working 'struct lconv'.

locale: Provide a working 'struct lconv'.

* lib/locale.in.h (lconv): Override if REPLACE_STRUCT_LCONV is 1.
* m4/locale_h.m4 (gl_LOCALE_H): Set REPLACE_STRUCT_LCONV to 1 if
'struct lconv' does not even contain decimal_point.
(gl_LOCALE_H_DEFAULTS): Initialize REPLACE_STRUCT_LCONV.
* modules/locale (Makefile.am): Substitute REPLACE_STRUCT_LCONV.
* tests/test-locale.c (main): Check that 'struct lconv' is complete.
* doc/posix-headers/locale.texi: Mention the problems with
'struct lconv'.
Reported by Gianluigi Tiesi <sherpya@netfarm.it>.