Trace is a bad choice of name for a module inside CIL. It prevents the OCAML toplevel from linking.
ocamlmktop -o cilmain -I ocamlfind -query cil nums.cma unix.cma str.cma cil.cma cilmain.cmo
File "/home/jrrk2/.opam/4.03.0/lib/cil/cil.cma(Trace)", line 1:
Warning 31: files /home/jrrk2/.opam/4.03.0/lib/cil/cil.cma(Trace) and
/home/jrrk2/.opam/4.03.0/lib/ocaml/compiler-libs/ocamltoplevel.cma(Trace) both define a module named Trace
Error: Some fatal warnings were triggered (1 occurrences)