If you redid all that work, and managed to avoid m4, it would still be a win! I believe m4 was used because some Unix shells at the time did not have functions

If you redid all that work, and managed to avoid m4, it would still be a win! I believe m4 was used because some Unix shells at the time did not have functions. All shells have functions now -- it's in POSIX. If you have shell functions, then you can have modularity without concatenating blocks of code. You can make a library of functions, and then allow people to reuse them. It could just be source lib_configure.sh detect_foo detect_bar etc. POSIX shell can do everything that m4 can, and it's MORE deployed, not less. (Related: some people missed that m4 never runs on the end user machine which compiles the source code -- only on the machine that makes the tarball from the repo.)