what is the correct entry for TAGS in OMakefile?

I have this in the top-level OMakefile:
------------------------------------------------------------------
TAGS:
	otags -r $(SRC_DIRS)
------------------------------------------------------------------
(where SRC_DIRS are also the dependencies of .SUBDIRS).
this is deficient for 2 reasons:

1. somehow otags tries to read non-ml files (despite "otags -h" claiming 
that suffixes are [.mli; .ml]) and fails on especially large ones:
------------------------------------------------------------------
otags: Unix error
   Syscall: stat
   Arg:     ./run/scan/top-20060612.log
   Error:   Value too large for defined data type
------------------------------------------------------------------

2. otags does not respect the pre-processors used for some files, 
resulting in many errors:
------------------------------------------------------------------
File "", line 15, characters 0-4:
Parse error: [implem] expected after [str_item_semi] (in [implem])
Uncaught exception: Stream.Error("[implem] expected after 
[str_item_semi] (in [implem])")
File "", line 12, characters 0-4:
Parse error: [implem] expected after [str_item_semi] (in [implem])
Uncaught exception: Stream.Error("[implem] expected after 
[str_item_semi] (in [implem])")
File "", line 12, characters 0-4:
Parse error: [implem] expected after [str_item_semi] (in [implem])
Uncaught exception: Stream.Error("[implem] expected after 
[str_item_semi] (in [implem])")
File "", line 10, characters 0-4:
Parse error: [implem] expected after [str_item_semi] (in [implem])
Uncaught exception: Stream.Error("[implem] expected after 
[str_item_semi] (in [implem])")
File "", line 11, characters 39-43:
Parse error: [implem] expected after [str_item_semi] (in [implem])
Uncaught exception: Stream.Error("[implem] expected after 
[str_item_semi] (in [implem])")
------------------------------------------------------------------
despite this, a valid TAGS file is created.
the preprocessing is specified in OMakeroot:
------------------------------------------------------------------
# Preprocess OCaml-files with a given OCaml-preprocessor
OCamlMakePPDeps(pps, names) =
   pp_cmos = $(addsuffix .cmo, $(pps))
   deps =
   foreach(pp_cmo, $(pp_cmos))
     deps += $(OCamlAddLibDir $(pp_cmos))
     export deps
   pp_cmd = camlp4o $(PREFIXED_OCAMLINCLUDES) $(pp_cmos)
   OCAMLPPFLAGS = -pp "$(pp_cmd)"
   OCAMLDEPFLAGS = $(OCAMLPPFLAGS)
   foreach(name, $(names))
     tgts = $(addsuffixes .cmi .cmo .cmx .o, $(name))
     scan_deps = scan-ocaml-$(name).ml scan-ocaml-$(name).mli
     Capture_deps(__ENV, \
       OCAMLPPFLAGS OCAMLDEPFLAGS, $(tgts), $(deps), $(scan_deps))
     tgts = $(addsuffix .auto.mli, $(name))
     Capture_deps(__ENV, \
       OCAMLPPFLAGS OCAMLDEPFLAGS, $(tgts), $(deps), $(scan_deps))
     export __ENV
   export __ENV
------------------------------------------------------------------
and in OMakefile:
------------------------------------------------------------------
OCamlMakePPDeps(pa_sexp_conv, foo bar baz)
------------------------------------------------------------------

so, how do I make ocaml TAGS?