Add build script and LIBC_PATH environ variable

Cargo.toml

@@ -13,6 +13,7 @@ A library for types and bindings to native C functions often found in libc or
 other common platform libraries.
 """
 include = ["Cargo.toml", "rust/src/liblibc/*"]
+build = "build.rs"
 
 [features]
 default = ["cargo-build"]
@@ -21,3 +22,5 @@ cargo-build = []
 [lib]
 name = "libc"
 path = "rust/src/liblibc/lib.rs"
+
+

build.rs

@@ -0,0 +1,7 @@
+use std::env;
+
+fn main() {
+    if let Ok(path) = env::var("LIBC_PATH") {
+        println!("cargo:rustc-link-search={}", path);
+    }
+}