Add support for 128-bit integers

Only compile in this support if rustc is new enough, otherwise continue
to omit the bindings for older-than-1.26 rustc

Closes #143
diff --git a/build.rs b/build.rs
index 30b5f05..3379da3 100644
--- a/build.rs
+++ b/build.rs
@@ -7,16 +7,20 @@
 
     let target = env::var("TARGET").unwrap();
 
-    if !enable_use_proc_macro(&target) {
-        return;
-    }
-    println!("cargo:rustc-cfg=use_proc_macro");
-
     let minor = match rustc_minor_version() {
         Some(n) => n,
         None => return,
     };
 
+    if minor >= 26 {
+        println!("cargo:rustc-cfg=u128");
+    }
+
+    if !enable_use_proc_macro(&target) {
+        return;
+    }
+    println!("cargo:rustc-cfg=use_proc_macro");
+
     // Rust 1.29 stabilized the necessary APIs in the `proc_macro` crate
     if minor >= 29 || cfg!(feature = "nightly") {
         println!("cargo:rustc-cfg=wrap_proc_macro");