Download xgboost from github at build time, instead of including as submodule.

XGBoost's source contains symlinks, which made packaging using cargo problematic.

Author: davechallis

.gitmodules

@@ -1,3 +0,0 @@
-[submodule "xgboost-sys/xgboost"]
-	path = xgboost-sys/xgboost
-	url = https://github.com/dmlc/xgboost.git

xgboost-sys/Cargo.toml

@@ -13,3 +13,4 @@ libc = "0.2"
 
 [build-dependencies]
 bindgen = "0.36"
+git2 = "0.7"

xgboost-sys/build.rs

@@ -1,13 +1,37 @@
 extern crate bindgen;
+extern crate git2;
 
 use std::process::Command;
 use std::env;
 use std::path::PathBuf;
-use std::fs::canonicalize;
+
+// Contains symlinks, so cannot be included as a git submodule and packaged with cargo. Downloaded
+// at build time instead.
+static XGBOOST_SRC: &str = "https://github.com/dmlc/xgboost.git";
+static XGBOOST_SPEC: &str = "refs/tags/v0.80";
 
 fn main() {
-    let target = env::var("TARGET").unwrap();
-    let xgb_root = canonicalize("xgboost").unwrap();
+    let target = env::var("TARGET").expect("Failed to read TARGET environment variable");
+
+    let xgb_root = std::path::Path::new("xgboost");
+
+    let repo = {
+        if !xgb_root.exists() {
+            match git2::Repository::clone_recurse(XGBOOST_SRC, "xgboost") {
+                Ok(repo) => repo,
+                Err(e) => panic!("failed to clone: {}", e),
+            }
+        } else {
+            match git2::Repository::open("xgboost") {
+                Ok(repo) => repo,
+                Err(e) => panic!("failed to open: {}", e),
+            }
+        }
+    };
+
+    repo.set_head(XGBOOST_SPEC).expect("failed to set head");
+
+    let xgb_root = xgb_root.canonicalize().unwrap();
 
     // TODO: allow for dynamic/static linking
     // TODO: check whether rabit should be built/linked