Make cross-compilation possible

README.md

@@ -14,6 +14,8 @@ The build script of crate will attempt to find the lib path of libpq using the
 following methods:
 
 * First, if the environment variable `PQ_LIB_DIR` is set, it will use its value
+* Second it will look for an environment variable in the format of `PQ_LIB_DIR_{TARGET}`
+where `{TARGET}` gets replaced by the Target environment variable set for cross-compilation
 * If the environment variable isn't set, it tries to use pkg-config to locate it.
 All the config options, such as `PKG_CONFIG_ALLOW_CROSS`, `PKG_CONFIG_ALL_STATIC`
 etc., of the crate [pkg-config](http://alexcrichton.com/pkg-config-rs/pkg_config/index.html)

build.rs

@@ -77,11 +77,30 @@ impl Display for LinkingOptions {
 }
 
 fn main() {
-    println!("cargo:rerun-if-env-changed=PQ_LIB_DIR");
     println!("cargo:rerun-if-env-changed=PQ_LIB_STATIC");
     println!("cargo:rerun-if-env-changed=TARGET");
 
-    if let Ok(lib_dir) = env::var("PQ_LIB_DIR") {
+    // if target is specified the more concrete pq_lib_dir overwrites a more general one
+    let lib_dir = if let Ok(target) = env::var("TARGET") {
+        let pq_lib_dir_for_target = format!("PQ_LIB_DIR_{}", target.to_ascii_uppercase().replace("-", "_"));
+        println!("cargo:rerun-if-env-changed={}", pq_lib_dir_for_target);
+        if let Ok(pg_lib_path) = env::var(pq_lib_dir_for_target.clone()) {
+            let path =  PathBuf::from(&pg_lib_path);
+            if !path.exists() {
+                panic!("Folder {:?} doesn't exist in the configured path: {:?}", pq_lib_dir_for_target, path);
+            }
+            println!("{:?} = {:?}", pq_lib_dir_for_target, pg_lib_path); // list in output for small debuggability
+            Ok(pg_lib_path)
+        }
+        else{
+            use_general_lib_dir()
+        }
+    }
+    else{
+        use_general_lib_dir()
+    };
+
+    if let Ok(lib_dir) = lib_dir {
         println!("cargo:rustc-link-search=native={}", lib_dir);
     } else if configured_by_pkg_config() {
         return // pkg_config does everything for us, including output for cargo
@@ -127,6 +146,19 @@ fn configured_by_vcpkg() -> bool {
     false
 }
 
+fn use_general_lib_dir() -> Result<String, VarError>{
+    println!("cargo:rerun-if-env-changed=PQ_LIB_DIR");
+    println!("PQ_LIB_DIR = {:?}", env::var("PQ_LIB_DIR"));
+
+    let pq_lib_dir = env::var("PQ_LIB_DIR");
+    if let Ok(pg_lib_path) = pq_lib_dir {
+        let path =  PathBuf::from(&pg_lib_path);
+        if !path.exists() {
+            panic!("Folder {:?} doesn't exist in the configured path: {:?}", "PQ_LIB_DIR", path);
+        }
+    }
+}
+
 fn pg_config_path() -> PathBuf {
     if let Ok(target) = env::var("TARGET") {
         let pg_config_for_target = &format!("PG_CONFIG_{}", target.to_ascii_uppercase().replace("-", "_"));