The attached patch extends autoconf-mode to:

- also fontify AU_, AX_, and gl_ macro calls as keywords;

- also recognise AH_, AS_, AU_, AX_, LT_, and gl_ macro calls as targets
  for beginning-of-defun, but not an opening square bracket in column 0;

- fontify the first argument of AC_DEFINE, AC_DEFINE_UNQUOTED, AC_SUBST,
  and AH_TEMPLATE with font-lock-variable-name-face instead of
  font-lock-function-name-face;

- also fontify the first argument of AH_VERBATIM, AM_CONDITIONAL, and
  AM_MISSING_PROG as a variable;

- fontify the first argument of AC_DEFUN, AC_DEFUN_ONCE, AU_ALIAS,
  AU_DEFUN, and function-like AC_DEFINE as a function;

- support backslashes, variable substitutions, and command substitutions
  with backticks in the first argument of AC_DEFINE_UNQUOTED; and

- fontify changequote only when it stands freely, not when it is part of
  m4_changequote (where either the whole macro name should be fontified,
  or no part of it).

WDYT?

Also, a question: should defun-prompt-regexp really allow for leading
indentation?  In Autoconf scripts of the form:

if ...

  ...
  AC_MSG_...
  ...
  AC_MSG_...
  ...

fi

it means that beginning-of-defun stops at every indented AC_ or AM_
macro call (and with the attached patch, at any macro call with a known
namespace).

Is this a feature (which allows jumping to the Autoconf parts in a sea
of shell code), or does it break from expected beginning-of-defun
semantics?

Thanks,
-- 
Basil