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>.