ftello: Add missing declaration on OSF/1 5.1. * lib/stdio.in.h (ftello): Test HAVE_DECL_FTELLO, not HAVE_FTELLO. * m4/ftello.m4 (gl_FUNC_FTELLO): Test whether ftello is declared. * m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Initialize HAVE_DECL_FTELLO. * modules/stdio (Makefile.am): Substitute HAVE_DECL_FTELLO. * doc/posix-functions/ftello.texi: Mention the OSF/1 5.1 problem.