autoconf

Provides fairly minimal font-lock, imenu and indentation support
for editing configure.ac files.  Only Autoconf syntax is processed.
There is no attempt to deal with shell text -- probably that will
always lose.

This is specialized for configure.ac files.  It doesn't inherit the
general M4 stuff from M4 mode.

There is also an autoconf-mode.el in existence.  That appears to be
for editing the Autoconf M4 source, rather than configure.ac files.