%% I should keep a separate list of files that are included-once which
   should also cache the realpaths.

@@ It would make sense to use $(call include-once,out_dir) in cxx/cxx-o.make
   etc.

@@ Need to check what is being framed.

@@ I wonder how much it costs to have the second expansion in implicit rule?

@@ Make executes each line in a separate shell. I wonder if using && is
   more efficient?

%% check make's default hash table size.

@@ would using `makefile: ' tecknique to prevent make from trying to
   match against pattern-rules help?

@@ Look for redundant $(eval ) calls (like I had in include-*).

@@ use head -n 1 instead of sed -e 1q