Currently, we have code in different folders like `flow/` and `fdbrpc/`
that should remain isolated. For example, `flow/` files should not
include functionality from any other modules. `fdbrpc/` files should
only be able to include functionality from itself and from `flow/`.
However, when creating a shared library, the linker doesn't complain
about undefined symbols -- this only happens when creating an
executable. Thus, for example, it is possible to forward declare an
`fdbclient` function in an `fdbrpc` file and then use it, and nothing
will break (when it should, because this is illegal).
This change adds dummy executables for a few modules (`flow`, `fdbrpc`,
`fdbclient`) that will cause a linker error if there are included
symbols which the linker can't resolve.
618f8455c4