Usage with Dune and Melange

[jchavarri]

When trying to use graphql-ppx with Melange, one runs into a couple of issues:

• ppx_runtime_libraries brings graphql_ppx.graphql_ppx_runtime, but this library is not compatible with melange modes
• there is no way to pick a "non-native" mode when using dune

For what it's worth, the diff we ended up with —and unblocked the compilation of the codebase we were trying to build— is here: master...jchavarri:melange.

One thing that was surprising is that once the native mode was set to false, it was not necessary to include the runtime for the code to compile successfully.

cc @anmonteiro @rgrinberg