Add way to specify target triple to run bindgen with (#8)

Expand interface to pass in the target triple for bindgen via the environment variable  `RUST_MBEDTLS_BINDGEN_TARGET`

mbedtls-sys/build/bindgen.rs

@@ -11,6 +11,7 @@ use bindgen;
 use std::fmt::Write as _;
 use std::fs::{self, File};
 use std::io::Write;
+use std::env;
 
 use crate::headers;
 
@@ -88,6 +89,7 @@ impl super::BuildConfig {
         }
 
         let mut cc = cc::Build::new();
+
         cc.include(&self.mbedtls_include)
         .flag(&format!(
             "-DMBEDTLS_CONFIG_FILE=\"{}\"",
@@ -116,6 +118,15 @@ impl super::BuildConfig {
             };
         }
 
+        // Bindgen utilizes libclang and the current `TARGET` to parse the C files.
+        // When the `TARGET` is custom, we need to override it so that bindgen
+        // finds the right stdlib headers.
+        // See https://docs.rust-embedded.org/embedonomicon/custom-target.html
+        // for more details.
+        if let Some(target) = env::var_os("RUST_MBEDTLS_BINDGEN_TARGET") {
+            env::set_var("TARGET", target);
+        }
+
         let bindings = bindgen::builder()
             .clang_args(cc.get_compiler().args().iter().map(|arg| arg.to_str().unwrap()))
             .header_contents("bindgen-input.h", &input)

mbedtls/src/lib.rs

@@ -21,7 +21,7 @@ extern crate bitflags;
 #[macro_use]
 extern crate serde_derive;
 // required explicitly to force inclusion at link time
-#[cfg(target_env = "sgx")]
+#[cfg(all(target_vendor = "fortanix", target_env = "sgx"))]
 extern crate rs_libc;
 
 #[macro_use]