This PR defines a cross-compiler `DEPRECATED(message)` macro. It must
be used as follows in Rust:
```rust
/// This is a documentation.
/// cbindgen:prefix=DEPRECATED(This is a deprecation message.")
pub extern "C" fn wasmer_foo() -> c_uint {
42
}
```
It will generate the following C header:
```c
/**
* This is a documentation.
*/
DEPRECATED("This is a deprecation message.")
unsigned int wasmer_foo();
```
And once this code is used by a C compiler, it will print something
like this (example from Clang):
```
…/test.c:…:…: error: 'wasmer_foo' is deprecated: This is a deprecation message. [-Werror,-Wdeprecated-declarations]
unsigned int x = wasmer_foo();
^
…/wasmer.h:…:…: note: 'wasmer_foo' has been explicitly marked deprecated here
DEPRECATED("This is a deprecation message.")
^
…/wasmer.h:…:…: note: expanded from macro 'DEPRECATED'
```
This is required for further deprecations.
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.
