New module 'wcslen'. * modules/wcslen: New file. * lib/wchar.in.h (wcslen): New declaration. * lib/wcslen.c: New file. * lib/wcslen-impl.h: New file, from libutf8 with modifications. * m4/wcslen.m4: New file. * m4/wchar_h.m4 (gl_WCHAR_H): Test whether wcslen is declared. (gl_WCHAR_H_DEFAULTS): Initialize GNULIB_WCSLEN, HAVE_WCSLEN. * modules/wchar (Makefile.am): Substitute GNULIB_WCSLEN, HAVE_WCSLEN. * tests/test-wchar-c++.cc: Test the declaration of wcslen. * doc/posix-functions/wcslen.texi: Mention the new module.