diff --git a/build.rs b/build.rs
new file mode 100644
index 0000000..985757a
--- /dev/null
+++ b/build.rs
@@ -0,0 +1,44 @@
+use std::env;
+use std::process::Command;
+use std::str::{self, FromStr};
+
+fn main(){
+    let minor = match rustc_minor_version() {
+        Some(minor) => minor,
+        None => return,
+    };
+
+    // const fn stabilized in Rust 1.31:
+    if minor >= 31 {
+        println!("cargo:rustc-cfg=bitflags_const_fn");
+    }
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = match env::var_os("RUSTC") {
+        Some(rustc) => rustc,
+        None => return None,
+    };
+
+    let output = match Command::new(rustc).arg("--version").output() {
+        Ok(output) => output,
+        Err(_) => return None,
+    };
+
+    let version = match str::from_utf8(&output.stdout) {
+        Ok(version) => version,
+        Err(_) => return None,
+    };
+
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+
+    let next = match pieces.next() {
+        Some(next) => next,
+        None => return None,
+    };
+
+    u32::from_str(next).ok()
+}
\ No newline at end of file