Add build.rs

Author: kngwyu

build.rs

@@ -0,0 +1,33 @@
+use std::process::{Command, Stdio};
+use std::str;
+fn is_python2() -> bool {
+    let script = r"
+import sys
+print(sys.version_info[0])
+";
+    let out = Command::new("python")
+        .args(&["-c", script])
+        .stderr(Stdio::inherit())
+        .output()
+        .expect("Failed to run the python interpreter");
+    let version = str::from_utf8(&out.stdout).unwrap();
+    version.starts_with("2")
+}
+
+fn cfg(python2: bool) {
+    if python2 {
+        println!("cargo:rustc-cfg=Py_2");
+    } else {
+        println!("cargo:rustc-cfg=Py_3");
+    }
+}
+
+fn main() {
+    if cfg!(feature = "python3") {
+        cfg(false);
+    } else if cfg!(feature = "python2") {
+        cfg(true);
+    } else {
+        cfg(is_python2());
+    }
+}