Makefile.in and rename it to Subdir.sd.mk, and add the appropriate
stuff to configure.ac, and fix everything up. Leave the existing
$(MAKE) -C for your existing subdirectories alone. Then you can
-convert individual subdirectories, or classes of subdirectories.
+convert individual subdirectories, or classes of subdirectories, at
+your leisure. (You must be /sure/ that each subdirectory will be
+entered only once at a time, but your existing recursive make descent
+system should already do that or you already have concurrency bugs.)
Aside from this, be very wary of any invocation of $(MAKE) anywhere.
This is a frequent source of concurrency bugs in recursive make build