You should use the small preprocessor that is provided with Caml Light.
Go to src/tools and type "make clprepro".
Change src/Makefile to uncomment the line
  # CPP=../../src/tools/clprepro -Dunix
and comment the line
  CPP=/lib/cpp -P -Dunix

There is a "#ifndef msdos" in lib/filename.mlp.  Exchange the contents
of the "then" and "else" parts and change the #ifndef into #ifdef.

Or simply remove everything between #else and #endif, as well as the
#ifndef line, since you're not under MSDOS.

Now do the "make world", "make bootstrap" etc.

-- Damien