System rocksdb and snappy libs through envvars

Variables: ROCKSDB_LIB_DIR, SNAPPY_LIB_DIR, ROCKSDB_STATIC and SNAPPY_STATIC

librocksdb-sys/build.rs

@@ -29,10 +29,7 @@ fn fail_on_empty_directory(name: &str) {
     }
 }
 
-fn build_rocksdb() {
-    println!("cargo:rerun-if-changed=build.rs");
-    println!("cargo:rerun-if-changed=rocksdb/");
-
+fn bindgen_rocksdb() {
     let bindings = bindgen::Builder::default()
         .header("rocksdb/include/rocksdb/c.h")
         .hide_type("max_align_t") // https://github.com/rust-lang-nursery/rust-bindgen/issues/550
@@ -44,7 +41,9 @@ fn build_rocksdb() {
     bindings
         .write_to_file(out_path.join("bindings.rs"))
         .expect("unable to write rocksdb bindings");
+}
 
+fn build_rocksdb() {
     let mut config = cc::Build::new();
     config.include("rocksdb/include/");
     config.include("rocksdb/");
@@ -148,9 +147,32 @@ fn build_snappy() {
     config.compile("libsnappy.a");
 }
 
+fn try_to_find_and_link_lib(lib_name: &str) -> bool {
+    if let Ok(lib_dir) = env::var(&format!("{}_LIB_DIR", lib_name)) {
+        println!("cargo:rustc-link-search=native={}", lib_dir);
+        let mode = match env::var_os(&format!("{}_STATIC", lib_name)) {
+            Some(_) => "static",
+            None => "dylib",
+        };
+        println!("cargo:rustc-link-lib={}={}", mode, lib_name.to_lowercase());
+        return true;
+    }
+    false
+}
+
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=rocksdb/");
+    println!("cargo:rerun-if-changed=snappy/");
+
     fail_on_empty_directory("rocksdb");
     fail_on_empty_directory("snappy");
+    bindgen_rocksdb();
+
+    if !try_to_find_and_link_lib("ROCKSDB") {
         build_rocksdb();
+    }
+    if !try_to_find_and_link_lib("SNAPPY") {
         build_snappy();
     }
+}