Make libsqlite3-sys's build script slightly more intelligent.

* If SQLITE3_LIB_DIR is present in the environment, we use that.
* If SQLITE3_LIB_DIR is not present, we try to use pkg-config.
* If SQLITE3_LIB_DIR is not present and pkg-config fails, we fall back
  to /usr/lib (if it exists).

libsqlite3-sys/build.rs

@@ -1,5 +1,26 @@
 extern crate pkg_config;
 
+use std::env;
+use std::fs;
+
 fn main() {
-    pkg_config::find_library("sqlite3").unwrap();
+    // Allow users to specify where to find SQLite.
+    let lib_dir = match env::var("SQLITE3_LIB_DIR") {
+        Ok(dir) => dir,
+        Err(_) => {
+            // See if pkg-config can do everything for us.
+            if pkg_config::find_library("sqlite3").is_ok() {
+                return
+            }
+
+            // Try to fall back to /usr/lib if pkg-config failed.
+            match fs::metadata("/usr/lib") {
+                Ok(ref attr) if attr.is_dir() => "/usr/lib".to_owned(),
+                _ => panic!("Could not find sqlite3. Try setting SQLITE3_LIB_DIR."),
+            }
+        },
+    };
+
+    println!("cargo:rustc-link-lib=sqlite3");
+    println!("cargo:rustc-link-search={}", lib_dir);
 }