It seems that form like this
```
.PHONY: instrumented-ocamlc
instrumented-ocamlc:
        $(MAKE) PPX="-ppx ./ppx/printexc_ppx" ocamlc
```
doesn't work at all. It produces an error of wrong `make` argument.
And if I'm not mistaken, following (and using PPX variable further) works as expected:
```
.PHONY: instrumented.ocamlc
instrumented.ocamlc: PPX=-ppx ./ppx/printexc_ppx
instrumented.ocamlc:
	$(MAKE) ocamlc
```
But the full picture of what is happening still does not add up.


I learned that `.SUFFIXES` produce implicit rules for creating files of source format to target.
For my situation it means that .cmo files produced from .ml files using
`$(CAMLC) $(COMPFLAGS) -c $<`
`CAMLC=$(BOOT_OCAMLC) -g -nostdlib -I boot -use-prims runtime/primitives`
`BOOT_OCAMLC = $(CAMLRUN) $(ROOTDIR)/boot/ocamlc`

And ocamlc is created by the
```
ocamlc: compilerlibs/ocamlcommon.cma compilerlibs/ocamlbytecomp.cma $(BYTESTART)
	$(CAMLC) $(LINKFLAGS) -compat-32 -o $@ $^
```
So I guess that I need to place PPX somewhere among these commands.

When I'm trying this:
```
.ml.cmo:
	$(CAMLC) $(COMPFLAGS) $(PPX) -c $<
```
or that:
`CAMLC=$(BOOT_OCAMLC) $(PPX) -g -nostdlib -I boot -use-prims runtime/primitives`

Then call `make world` and `make instrumented.ocamlc`, I'm getting
```
make ocamlc
make[1]: entering the directory «/home/lereena/ocaml»
make -C utils config.ml
make[2]: entering the directory «/home/lereena/ocaml/utils»
make[2]: «config.ml» does not require updating.
make[2]: entering the directory «/home/lereena/ocaml/utils»
make[1]: entering the directory «/home/lereena/ocaml»
```

It seems that it is necessary to inform the project somewhere that it is necessary to recompile ocamlc,
but it is not very clear where, and whether this is a problem at all.