`wasmer-runtime` has a `debug` and a `llvm` features. Let's mirror
them in `wasmer-runtime-c-api` so that the user can, for instance,
compile with the LLVM backend.
When compiling `wasmer-runtime-c-api` as a dependency of another
crate, `rustc` emits this warning:
```
warning: The package `wasmer_runtime_c_api` provides no linkable
target. The compiler might raise an error while compiling
`foo`. Consider adding 'dylib' or 'rlib' to key `crate-type` in
`wasmer_runtime_c_api`'s Cargo.toml. This warning might turn into a
hard error in the future.
```
To solve this issue, the `rlib` type has been added to the
`crate-type` list.
This patch removes the `WASM_EMSCRIPTEN_GENERATE_C_API_HEADERS`
flag. Consequently, the C header files will be generated for each
build.
The `generate-c-api-headers` feature is also removed, since it becomes useless.
When requiring `wasmer-runtime-c-api`, I get the following conflicts:
```
error: failed to select a version for `serde_derive`.
... required by package `cbindgen v0.7.1`
... which is depended on by `wasmer-runtime-c-api v0.2.1`
... which is depended on by `foo v0.1.0 (/private/tmp/foo)`
versions that meet the requirements `= 1.0.58` are: 1.0.58
all possible versions conflict with previously selected packages.
previously selected package `serde_derive v1.0.89`
... which is depended on by `wasmer-runtime-core v0.2.1`
... which is depended on by `wasmer-runtime-c-api v0.2.1`
... which is depended on by `foo v0.1.0 (/private/tmp/foo)`
failed to select a version for `serde_derive` which could resolve this conflict
```
This issue resolves by updating `cbindgen` to 0.8.
