Would it make sense to replace the fat list of *.m4 files with a $(wildcard m4/*.m4) ?
It would save the need to edit this list every time we add a new file to the directory.
The same might be relevant to various *.[ch] lists below.
Only valid if we presume GNU make everywhere the generated make-file is to be used, though.
(If not, perhaps auto-make has some equivalent function ?)