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.