Fix compilation using OCaml 4.01.0

The `QuickCheck_util` module of the `ocaml-quickcheck` package defines a `(|>)` operator, which is also introduced in the OCaml 4.01.0 `Pervasives`. Due to this, when `open`'ing the `QuickCheck_util` module, a warning is generated.

Since this package is compiled using warnings-as-errors, this breaks the build.

These commits work around this situation by using the `optcomp` syntax extension in order to conditionally define the operator. Another less intrusive approach could be to rename the operator to something else, but I'm not sure to which extent this operator is part of the public API of this package.