wcstok: Work around wrong signature on native Windows. * lib/wchar.in.h (wcstok): Override when REPLACE_WCSTOK is 1. * m4/wcstok.m4 (gl_FUNC_WCSTOK): Check for signature of wcstok. Set REPLACE_WCSTOK. * m4/wchar_h.m4 (gl_WCHAR_H_DEFAULTS): Initialize REPLACE_WCSTOK. * modules/wchar (Makefile.am): Substitute REPLACE_WCSTOK. * modules/wcstok (Depends-on, configure.ac): Consider REPLACE_WCSTOK. * doc/posix-functions/wcstok.texi: Mention the problem.