Introduce target-specific env vars

The logic is stolen from cmake-rs, and it is important to
follow it as we will need to look for CMAKE_TOOLCHAIN_FILE
the same way cmake-rs does.

When checking for env variable BORING_BSSL_PATH during a
cross build for target x86_64-unknown-linux-gnu, boring-sys
build script will attempt to read:

  BORING_BSSL_PATH_x86_64-unknown-linux-gnu
  BORING_BSSL_PATH_x86_64_unknown_linux_gnu
  TARGET_BORING_BSSL_PATH
  BORING_BSSL_PATH

boring-sys/build/config.rs

@@ -43,7 +43,11 @@ impl Config {
         let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap();
 
         let features = Features::from_env();
-        let env = Env::from_env(features.fips || features.fips_link_precompiled);
+        let env = Env::from_env(
+            &host,
+            &target,
+            features.fips || features.fips_link_precompiled,
+        );
 
         let config = Self {
             manifest_dir,
@@ -107,19 +111,31 @@ impl Features {
 }
 
 impl Env {
-    fn from_env(is_fips_like: bool) -> Self {
+    fn from_env(target: &str, host: &str, is_fips_like: bool) -> Self {
         const NORMAL_PREFIX: &str = "BORING_BSSL";
         const FIPS_PREFIX: &str = "BORING_BSSL_FIPS";
 
+        let target_with_underscores = target.replace('-', "_");
+
+        // Logic stolen from cmake-rs.
+        let target_var = |name: &str| {
+            let kind = if host == target { "HOST" } else { "TARGET" };
+
+            var(&format!("{}_{}", name, target))
+                .or_else(|| var(&format!("{}_{}", name, target_with_underscores)))
+                .or_else(|| var(&format!("{}_{}", kind, name)))
+                .or_else(|| var(name))
+        };
+
         let boringssl_var = |name: &str| {
             // The passed name is the non-fips version of the environment variable,
             // to help look for them in the repository.
             assert!(name.starts_with(NORMAL_PREFIX));
 
             if is_fips_like {
-                var(&name.replace(NORMAL_PREFIX, FIPS_PREFIX))
+                target_var(&name.replace(NORMAL_PREFIX, FIPS_PREFIX))
             } else {
-                var(name)
+                target_var(name)
             }
             .map(PathBuf::from)
         };
@@ -129,9 +145,9 @@ impl Env {
             include_path: boringssl_var("BORING_BSSL_INCLUDE_PATH"),
             source_path: boringssl_var("BORING_BSSL_SOURCE_PATH"),
             precompiled_bcm_o: boringssl_var("BORING_BSSL_PRECOMPILED_BCM_O"),
-            debug: var("DEBUG"),
-            opt_level: var("OPT_LEVEL"),
-            android_ndk_home: var("ANDROID_NDK_HOME").map(Into::into),
+            debug: target_var("DEBUG"),
+            opt_level: target_var("OPT_LEVEL"),
+            android_ndk_home: target_var("ANDROID_NDK_HOME").map(Into::into),
         }
     }
 }