tolower_l: New module. * lib/ctype.in.h: (tolower_l): New declaration. * lib/tolower_l.c: New file. * lib/to_l-impl.h: New file. * m4/tolower_l.m4: New file. * m4/ctype_h.m4 (gl_CTYPE_H): Test for tolower_l. (gl_CTYPE_H_REQUIRE_DEFAULTS): Initialize GNULIB_TOLOWER_L. (gl_CTYPE_H_DEFAULTS): Initialize HAVE_TOLOWER_L. * modules/ctype-h (Makefile.am): Substitute GNULIB_TOLOWER_L, HAVE_TOLOWER_L. * modules/tolower_l: New file. * tests/test-ctype-h-c++.cc: Check declaration of tolower_l. * doc/posix-functions/tolower_l.texi: Mention the new module.