wctype-h: Add support for Minix. * m4/wctype_h.m4 (gl_WCTYPE_H): Test whether towlower is declared. Set REPLACE_TOWLOWER. * modules/wctype-h (Makefile.am): Substitute REPLACE_TOWLOWER. * lib/wctype.in.h (towlower, towupper): Test REPLACE_TOWLOWER, not REPLACE_ISWCNTRL.