Good evening all-- The attached file adds m4 support.  It was tested against org-9.1.7 and used ob-sed.el and ob-shell.el for inspiration.  Both code execution and tangling have been tested with simple inputs as well as tables (easiest way to verify correctness is to inspect the tangled output).  It was good fun for an evening project (converting the tables to m4 list syntax was the trickiest part and required a foray into ob-shell.el; pcase is a nifty function). When putting it together, I debated between putting the definitions representing variables in as -DNAME=value pairs in the command-line or putting them in the body of the generate file.  Even though it's a bit uglier, I decided to put them in the body of the file as it makes the generated file capable of being run by hand. Finally, thanks for the work on org-mode and babel.  Vanilla org-mode has made my meeting notes are significantly easier and PDF exports look terrific.  Likewise, while I've written numerous documents using R/knitr, I appreciate having a literate environment that supports additional languages. --Brad