wctype: Avoid compilation error on IRIX 6.5.30. * lib/wctype.in.h (iswblank): Declare with a replacement if REPLACE_ISWBLANK is set. * m4/wctype_h.m4 (gl_WCTYPE_H): Check also whether iswblank is declared. Set REPLACE_ISWBLANK. * modules/wctype (Makefile.am): Substitute REPLACE_ISWBLANK. * doc/posix-functions/iswblank.texi: Mention the IRIX 6.5.30 problem. * doc/posix-headers/wctype.texi: Likewise. Reported by Tom G. Christensen <tgc@jupiterrise.com>.