Where there are dependencies between subdirectories, each Subdir.mk.in
can simply refer to files in other subdirectories directly.
-Per-directory targets, "recursion"
-----------------------------------
+Invocation, "recursive" per-directory targets
+---------------------------------------------
+
+Arrangements are made so that when you run `make foo' in a
+subdirectory, it is like running the whole toplevel makefile, from the
+toplevel, as `make subdir/foo'. If `subdir/foo' is a file that might
+be built, that builds it.
+
+But `foo' can also be a conventional target like `all'.
Each subdirectory has its own `all' target. For example a
subdirectory `src' has a target `src/all'. The rules for these are
targets may be in run in parallel: there is only one `make'
invocation at a time.
-There may also be other
+You can define other per-directory recursive targets too: simply
+mention (usually, by setting) the variable &TARGETS_zonk, or whatever.
+This will create a src/zonk target. (&TARGETS is magic.)
+Unlike `all', these other targets only exist in areas of the project
+where at least something mentions them. So for example, if
+&TARGETS_zonk is mentioned in src but not lib, `make zonk' in
+lib will fail. If you want to make a target exist everywhere,
+mention its name in Perdir.mk.in (see below).
+
+Perdir.mk.in, inclusion
+-----------------------
+
+The file Perdir.mk.in in the toplevel of fthe source is automatically
+processed after each individual directory's Subdir.mk.in, and the
+&-substituted contents therefore appear once for each subdirectory.
+
+This lets you do per-directory boilerplate. Some useful boilerplate
+is already provided in subdirmk, for you to reference like this:
+ &:include subdirmk/cdeps.mk.in
+ &:include subdirmk/clean.mk.in
+
+Note that you must use &:include, which is an include processed during
+the generation of the per-directory Subdir.mk files. That ensures
+that the contents of these files is replicated, with appropriate
+per-directory substitutions, for each directory.
+
+Global definitions
+------------------
+
+If want to set global variables, such as CC
+
+
+
+ subdirmk/cdeps.mk.in
+ subdirmk/cdeps.mk.in
+