Unable to build CIL due to unbound module Big_int

[charmoniumQ]

Running

cd cil
./configure
make

fails with

make[1]: Leaving directory '/home/sam/Downloads/test2/test/crest/cil'
+ ocamlc.opt -c -I src -I ocamlutil -I src/frontc -I src/ext -I src/ext/pta -o src/cilint.cmi src/cilint.mli
+ ocamlc.opt -c -I src -I ocamlutil -I src/frontc -I src/ext -I src/ext/pta -o src/cilint.cmi src/cilint.mli
File "src/cilint.mli", line 6, characters 36-51:
6 | type cilint = Small of int | Big of Big_int.big_int
                                        ^^^^^^^^^^^^^^^
Error: Unbound module Big_int
Command exited with code 2.
make: *** [Makefile:79: _build/src/cil.cma] Error 10

Note, I am using OCaml version 4.12.1.