Clang can emulate GNUC or MSVC behaviors. So instead of detecting
whether Clang is used to compile the code, it's better to detect the
targeted features are available.
Code proposed by @nlewycky, thanks!
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.
