feat(runtime-c-api) Generate the C header file in OUT_DIR.

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.

lib/runtime-c-api/build.rs

@@ -1,7 +1,7 @@
 #[cfg(feature = "generate-c-api-headers")]
 extern crate cbindgen;
 
-use std::env;
+use std::{env, path::Path};
 
 static CAPI_ENV_VAR: &str = "WASM_EMSCRIPTEN_GENERATE_C_API_HEADERS";
 
@@ -14,6 +14,12 @@ fn main() {
 #[cfg(feature = "generate-c-api-headers")]
 fn build() {
     let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let out_path = Path::new(&out_dir);
+    let mut wasmer_h = out_path.to_path_buf();
+    wasmer_h.push("wasmer.h");
+    let mut wasmer_hh = out_path.to_path_buf();
+    wasmer_hh.push("wasmer.hh");
 
     use cbindgen::Language;
     cbindgen::Builder::new()
@@ -22,15 +28,15 @@ fn build() {
         .with_include_guard("WASMER_H")
         .generate()
         .expect("Unable to generate C bindings")
-        .write_to_file("wasmer.h");
+        .write_to_file(wasmer_h);
 
     cbindgen::Builder::new()
         .with_crate(crate_dir)
         .with_language(Language::Cxx)
         .with_include_guard("WASMER_H")
         .generate()
         .expect("Unable to generate C++ bindings")
-        .write_to_file("wasmer.hh");
+        .write_to_file(wasmer_hh);
 }
 
 #[cfg(not(feature = "generate-c-api-headers"))]