Add syntax highlighting for GNU Autotools

[dj1vs]

Motivation
I am writing an article in Obsidian about GNU Autotools and I found out that there is no syntax highlighting available.
GNU Autotools is popular, and a lot of developers still use it today.

Description
By "GNU Autotools syntax highlighting" I mean highlighting for autoconf files (.ac) and automake files (.am).

Alternatives
I think it would be OK to just implement M4 syntax highlighting because it is more common macros processor, on which autoconf syntax is built.