AC_PROVIDE_IFELSE, AC_FATAL, AC_DEFUN_ONCE not documented

[Karl Berry]

It seems AC_PROVIDE_IFELSE, AC_FATAL, AC_DEFUN_ONCE are not mentioned in
the autoconf manual.

I noticed them in Akim's ChangeLog entry from 2000, FWIW.
It makes me wonder about somehow dumping all AC_* names and comparing to
the manual ...

2000-11-01  Akim Demaille  <a...@epita.fr>

        Move the `defun' handling into M4sugar.
        [...]
        (AC_DEFUN, AC_DEFUN_ONCE, _AC_REQUIRE, AC_REQUIRE)
        AC_PROVIDE_IFELSE, AC_FATAL): Reimplement atop M4sugar.