sh-autoconf.el extends ShMode with an autoconf shell flavour designed for editing Autoconf scripts and acinclude.m4 etc macro files. Get it from

The default for autoconf files is AutoconfMode which is geared towards autoconf macro calls. Whether you prefer it or sh-autoconf depends mainly on how much shell code you’re writing in amongst the macros. A simple configure might be all macros, a big complicated one inevitably ends up with lots of shell.