"ocaml_beginners"::[] Conditional compilation

Fellow Camels,

How does one handle conditional compilation in OCaml?  I want certain
pieces of debugging code to be included only if a certain environment
variable is set.  This sort of thing is really easy to do in the C
preprocessor with #ifdefs.

And yes, I know I can use cpp also with OCaml, but I was wondering
if there was a more "bactrian" way of doing this.

Thanks,
C.S.