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.
This patch changes the directory where the C header files are
generated, from `CARGO_MANIFEST_DIR` to `OUT_DIR`. The goal is: When
`wasm-runtime-c-api` is used as a dependency of another Rust project,
then the C header files are accessible in the `target/` directory
(i.e. the `OUT_DIR`).
Also, since `rustc` has a `--out-dir` directory, it increases the
flexibility of this approach: The user can generate the C header files
where she wants.
