Fail if RocksDB or Snappy source is missing

README.md

@@ -11,3 +11,7 @@ Feedback and pull requests welcome!  If a particular feature of RocksDB is impor
 [dependencies]
 rocksdb = "0.6.0"
 ```
+
+This binding is statically linked with a specific version of RocksDB. If you want to build it yourself, make sure you've also cloned the RocksDB and Snappy submodules:
+
+    git submodule update --init --recursive

librocksdb-sys/build.rs

@@ -1,5 +1,7 @@
 extern crate gcc;
 
+use std::fs;
+
 fn link(name: &str, bundled: bool) {
     use std::env::var;
     let target = var("TARGET").unwrap();
@@ -13,6 +15,14 @@ fn link(name: &str, bundled: bool) {
     }
 }
 
+fn fail_on_empty_directory(name: &str) {
+    if fs::read_dir(name).unwrap().count() == 0 {
+        println!("The `{}` directory is empty, did you forget to pull the submodules?", name);
+        println!("Try `git submodule update --init --recursive`");
+        panic!();
+    }
+}
+
 fn build_rocksdb() {
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-changed=rocksdb/");
@@ -118,6 +128,8 @@ fn build_snappy() {
 }
 
 fn main() {
+    fail_on_empty_directory("rocksdb");
+    fail_on_empty_directory("snappy");
     build_rocksdb();
     build_snappy();
 }